OPINION OF THE COURT NORTON V. SOUTHERN UTAH WILDERNESS ALLIANCE 542 U. S. ____ (2004) SUPREME COURT OF THE UNITED STATES NO. 03-101 GALE NORTON, SECRETARY OF THE INTERIOR,
et al., PETITIONERS v. SOUTHERN UTAH WILDERNESS
ALLIANCE et al.

															on writ of certiorari to the united states court of
appeals for the tenth circuit

															[June 14, 2004]

															   Justice Scalia delivered the
opinion of the Court.

															   In this case, we must decide
whether the authority of a federal court under the Administrative
Procedure Act (APA) to “compel agency action unlawfully withheld or
unreasonably delayed,” 5 U. S. C. §706(1), extends to the
review of the United States Bureau of Land Management’s stewardship
of public lands under certain statutory provisions and its own
planning documents.

															I

															   Almost half the State of Utah,
about 23 million acres, is federal land administered by the Bureau
of Land Management (BLM), an agency within the Department of
Interior. For nearly 30 years, BLM’s management of public lands has
been governed by the Federal Land Policy and Management Act of 1976
(FLPMA), 90 Stat. 2744, 43 U. S. C. §1701 et seq. ,
which “established a policy in favor of retaining public lands for
multiple use management.” Lujan v. National Wildlife
Federation, 497 U. S. 871 , 877 (1990). “Multiple
use management” is a deceptively simple term that describes the
enormously complicated task of striking a balance among the many
competing uses to which land can be put, “including, but not
limited to, recreation, range, timber, minerals, watershed,
wildlife and fish, and [uses serving] natural scenic, scientific
and historical values.” 43 U. S. C. §1702(c). A second
management goal, “sustained yield,” requires BLM to control
depleting uses over time, so as to ensure a high level of valuable
uses in the future. §1702(h). To these ends, FLPMA establishes a
dual regime of inventory and planning. Sections 1711 and 1712,
respectively, provide for a comprehensive, ongoing inventory of
federal lands, and for a land use planning process that
“project[s]” “present and future use,” §1701(a)(2), given the
lands’ inventoried characteristics.

															   Of course not all uses are
compatible. Congress made the judgment that some lands should be
set aside as wilderness at the expense of commercial and
recreational uses. A pre-FLPMA enactment, the Wilderness Act of
1964, 78 Stat. 890, provides that designated wilderness areas,
subject to certain exceptions, “shall [have] no commercial
enterprise and no permanent road,” no motorized vehicles, and no
manmade structures. 16 U. S. C. §1133(c). The designation of a
wilderness area can be made only by Act of Congress, see 43 U.
S. C. §1782(b).

															   Pursuant to §1782, the Secretary of the
Interior has identified so-called “wilderness study areas” (WSAs),
roadless lands of 5,000 acres or more that possess “wilderness
characteristics,” as determined in the Secretary’s land inventory.
§1782(a); see 16 U. S. C. §1131(c). As the name suggests, WSAs
(as well as certain wild lands identified prior to the passage of
FLPMA) have been subjected to further examination and public
comment in order to evaluate their suitability for designation as
wilderness. In 1991, out of 3.3 million acres in Utah that had been
identified for study, 2 million were recommended as suitable for
wilderness designation. 1 U. S. Dept. of Interior, BLM, Utah
Statewide Wilderness Study Report 3 (Oct. 1991). This
recommendation was forwarded to Congress, which has not yet acted
upon it. Until Congress acts one way or the other, FLPMA provides
that “the Secretary shall continue to manage such lands . . . in a
manner so as not to impair the suitability of such areas for
preservation as wilderness.” 43 U. S. C. §1782(c). This
nonimpairment mandate applies to all WSAs identified under §1782,
including lands considered unsuitable by the Secretary. See
§§1782(a), (b); App. 64 (BLM Interim Management Policy for Lands
Under Wilderness Review).

															   Aside from identification of WSAs, the main
tool that BLM employs to balance wilderness protection against
other uses is a land use plan—what BLM regulations call a “resource
management plan.” 43 CFR §1601.0–5(k) (2003). Land use plans,
adopted after notice and comment, are “designed to guide and
control future management actions,” §1601.0–2. See 43 U. S. C.
§1712; 43 CFR §1610.2 (2003). Generally, a land use plan describes,
for a particular area, allowable uses, goals for future condition
of the land, and specific next steps. §1601.0–5(k). Under FLPMA,
“[t]he Secretary shall manage the public lands under principles of
multiple use and sustained yield, in accordance with the land use
plans … when they are available.” 43 U. S. C. §1732(a).

															   Protection of wilderness has come into
increasing conflict with another element of multiple use,
recreational use of so-called off-road vehicles (ORVs), which
include vehicles primarily designed for off-road use, such as
lightweight, four-wheel “all-terrain vehicles,” and vehicles
capable of such use, such as sport utility vehicles. See 43 CFR
§8340.0–5(a) (2003). According to the United States Forest
Service’s most recent estimates, some 42 million Americans
participate in off-road travel each year, more than double the
number two decades ago. H. Cordell, Outdoor Recreation for 21st
Century America 40 (2004). United States sales of all-terrain
vehicles alone have roughly doubled in the past five years,
reaching almost 900,000 in 2003. See Tanz, Making Tracks, Making
Enemies, N. Y. Times, Jan. 2, 2004, p. F1, col. 5; Discover
Today’s Motorcycling, Motorcycle Industry Council, Press Release,
Feb. 13, 2004, http://www.motorcycles.org (all Internet materials
as visited June 4, 2004, and available in Clerk of Court’s case
file). The use of ORVs on federal land has negative environmental
consequences, including soil disruption and compaction, harassment
of animals, and annoyance of wilderness lovers. See Brief for
Natural Resources Defense Council et al. as Amici
Curiae 4–7, and studies cited therein. Thus, BLM faces a
classic land use dilemma of sharply inconsistent uses, in a context
of scarce resources and congressional silence with respect to
wilderness designation.

															   In 1999, respondents Southern Utah Wilderness
Alliance and other organizations (collectively SUWA) filed this
action in the United States District Court for Utah against
petitioners BLM, its Director, and the Secretary. In its second
amended complaint, SUWA sought declaratory and injunctive relief
for BLM’s failure to act to protect public lands in Utah from
damage caused by ORV use. SUWA made three claims that are relevant
here: (1) that BLM had violated its nonimpairment obligation under
§1782(a) by allowing degradation in certain WSAs; (2) that BLM had
failed to implement provisions in its land use plans relating to
ORV use; (3) that BLM had failed to take a “hard look” at whether,
pursuant to the National Environmental Policy Act of 1969 (NEPA),
83 Stat. 852, 42 U. S. C. §4321 et seq ., it should
undertake supplemental environmental analyses for areas in which
ORV use had increased. SUWA contended that it could sue to remedy
these three failures to act pursuant to the APA’s provision of a
cause of action to “compel agency action unlawfully withheld or
unreasonably delayed.” 5 U. S. C. §706(1).

															   The District Court entered a dismissal with
respect to the three claims. A divided panel of the Tenth Circuit
reversed. 301 F. 3d 1217 (2002). The majority acknowledged
that under §706(1), “federal courts may order agencies to act only
where the agency fails to carry out a mandatory, nondiscretionary
duty.” Id. , at 1226. It concluded, however, that BLM’s
nonimpairment obligation was just such a duty, and therefore BLM
could be compelled to comply. Under similar reasoning, it reversed
the dismissal with respect to the land use plan claim; and likewise
reversed dismissal of the NEPA claim. We granted certiorari. 540 U. S. 980 (2003).

															II

															   All three claims at issue here
involve assertions that BLM failed to take action with respect to
ORV use that it was required to take. Failures to act are sometimes
remediable under the APA, but not always. We begin by considering
what limits the APA places upon judicial review of agency
inaction.

															   The APA authorizes suit by “[a]
person suffering legal wrong because of agency action, or adversely
affected or aggrieved by agency action within the meaning of a
relevant statute.” 5 U. S. C. §702. Where no other statute
provides a private right of action, the “agency action” complained
of must be “ final agency action.” §704 (emphasis added).
“Agency action” is defined in §551(13) to include “the whole or a
part of an agency rule, order, license, sanction, relief, or the
equivalent or denial thereof, or failure to act. ”
(Emphasis added.) The APA provides relief for a failure to act in
§706(1): “The reviewing court shall … compel agency action
unlawfully withheld or unreasonably delayed.”

															   Sections 702, 704, and 706(1) all insist upon
an “agency action,” either as the action complained of (in §§702
and 704) or as the action to be compelled (in §706(1)). The
definition of that term begins with a list of five categories of
decisions made or outcomes implemented by an agency—“agency rule,
order, license, sanction [or] relief.” §551(13). All of those
categories involve circumscribed, discrete agency actions, as their
definitions make clear: “an agency statement of … future effect
designed to implement, interpret, or prescribe law or policy”
(rule); “a final disposition … in a matter other than rule making”
(order); a “permit … or other form of permission” (license); a
“prohibition … or taking [of] other compulsory or restrictive
action” (sanction); or a “grant of money, assistance, license,
authority,” etc., or “recognition of a claim, right, immunity,”
etc., or “taking of other action on the application or petition of,
and beneficial to, a person” (relief). §§551(4), (6), (8), (10),
(11).

															   The terms following those five categories of
agency action are not defined in the APA: “or the equivalent or
denial thereof, or failure to act.” §551(13). But an “equivalent …
thereof” must also be discrete (or it would not be equivalent), and
a “denial thereof” must be the denial of a discrete listed action
(and perhaps denial of a discrete equivalent).

															   The final term in the definition, “failure to
act,” is in our view properly understood as a failure to take an agency action —that is, a failure to take one of the agency
actions (including their equivalents) earlier defined in §551(13).
Moreover, even without this equation of “act” with “agency action”
the interpretive canon of ejusdem generis would attribute
to the last item (“failure to act”) the same characteristic of
discreteness shared by all the preceding items. See, e.g., Washington State Dept. of Social and Health Servs. v. Guardianship Estate of Keffeler, 537 U. S. 371 , 384–385
(2003). A “failure to act” is not the same thing as a “denial.” The
latter is the agency’s act of saying no to a request; the former is
simply the omission of an action without formally rejecting a
request—for example, the failure to promulgate a rule or take some
decision by a statutory deadline. The important point is that a
“failure to act” is properly understood to be limited, as are the
other items in §551(13), to a discrete action.

															   A second point central to the analysis of the
present case is that the only agency action that can be compelled
under the APA is action legally required . This limitation
appears in §706(1)’s authorization for courts to “compel agency
action unlawfully withheld.”[ Footnote 1 ] In this regard the APA carried forward the
traditional practice prior to its passage, when judicial review was
achieved through use of the so-called prerogative writs—principally
writs of mandamus under the All Writs Act, now codified at 28 U.
S. C. §1651(a). The mandamus remedy was normally limited to
enforcement of “a specific, unequivocal command,” ICC v. New York, N. H. & H. R. Co., 287 U. S. 178 , 204
(1932), the ordering of a “ ‘precise, definite act … about
which [an official] had no discretion whatever,’ ” United
States ex rel. Dunlap v. Black, 128 U. S. 40 , 46 (1888)
(quoting Kendall v. United States ex rel. Stokes, 12 Pet. 524, 613 (1838)). See also ICC v. United
States ex rel. Humboldt S. S. Co., 224 U. S. 474 , 484
(1912). As described in the Attorney General’s Manual on the APA, a
document whose reasoning we have often found persuasive, see, e.g., Darby v. Cisneros, 509 U. S. 137 , 148,
n. 10 (1993); Chrysler Corp. v. Brown, 441 U. S. 281 , 302, n.
31 (1979); Vermont Yankee Nuclear Power Corp. v. Natural Resources Defense Council, Inc., 435 U. S. 519 , 546
(1978), §706(1) empowers a court only to compel an agency “to
perform a ministerial or non-discretionary act,” or “to take action
upon a matter, without directing how it shall act.”
Attorney General’s Manual on the Administrative Procedure Act 108
(1947) (emphasis added). See also L. Jaffe, Judicial Control of
Administrative Action 372 (1965); K. Davis, Administrative Law
§257, p. 925 (1951).

															   Thus, a claim under §706(1) can proceed only
where a plaintiff asserts that an agency failed to take a discrete agency action that it is required to
take . These limitations rule out several kinds of challenges.
The limitation to discrete agency action precludes the kind of
broad programmatic attack we rejected in Lujan v. National Wildlife Federation, 497 U. S. 871 (1990). There we
considered a challenge to BLM’s land withdrawal review program,
couched as unlawful agency “action” that the plaintiffs wished to
have “set aside” under §706(2).[ Footnote 2 ] Id. , at 879. We concluded that the
program was not an “agency action”:

															   “[R]espondent cannot seek wholesale improvement of this program by court decree,
rather than in the offices of the Department or the halls of
Congress, where programmatic improvements are normally made. Under
the terms of the APA, respondent must direct its attack against
some particular ‘agency action’ that causes it harm.” Id. ,
at 891 (emphasis in original).

															The plaintiffs in National Wildlife
Federation would have fared no better if they had
characterized the agency’s alleged “failure to revise land use
plans in proper fashion” and “failure to consider multiple use,” ibid ., in terms of “agency action unlawfully withheld”
under §706(1), rather than agency action “not in accordance with
law” under §706(2).

															   The limitation to required agency
action rules out judicial direction of even discrete agency action
that is not demanded by law (which includes, of course, agency
regulations that have the force of law). Thus, when an agency is
compelled by law to act within a certain time period, but the
manner of its action is left to the agency’s discretion, a court
can compel the agency to act, but has no power to specify what the
action must be. For example, 47 U. S. C. §251(d)(1), which
required the Federal Communications Commission “to establish
regulations to implement” interconnection requirements “[w]ithin 6
months” of the date of enactment of the Telecommunications Act of
1996, would have supported a judicial decree under the APA
requiring the prompt issuance of regulations, but not a judicial
decree setting forth the content of those regulations.

															III

															A

															   With these principles in mind, we
turn to SUWA’s first claim, that by permitting ORV use in certain
WSAs, BLM violated its mandate to “continue to manage [WSAs] … in a
manner so as not to impair the suitability of such areas for
preservation as wilderness,” 43 U. S. C. §1782(c). SUWA relies
not only upon §1782(c) but also upon a provision of BLM’s Interim
Management Policy for Lands Under Wilderness Review, which
interprets the nonimpairment mandate to require BLM to manage WSAs
so as to prevent them from being “degraded so far, compared with
the area’s values for other purposes, as to significantly constrain
the Congress’s prerogative to either designate [it] as wilderness
or release it for other uses.” App. 65.

															   Section 1782(c) is mandatory as
to the object to be achieved, but it leaves BLM a great deal of
discretion in deciding how to achieve it. It assuredly does not
mandate, with the clarity necessary to support judicial action
under §706(1), the total exclusion of ORV use.

															   SUWA argues that §1782 does contain a
categorical imperative, namely the command to comply with the
nonimpairment mandate. It contends that a federal court could
simply enter a general order compelling compliance with that
mandate, without suggesting any particular manner of compliance. It
relies upon the language from the Attorney General’s Manual quoted
earlier, that a court can “take action upon a matter, without
directing how [the agency] shall act,” and upon language in a case
cited by the Manual noting that “mandamus will lie … even though
the act required involves the exercise of judgment and discretion.” Safeway Stores v. Brown , 138 F. 2d 278, 280
(Emerg. Ct. App. 1943). The action referred to in these excerpts,
however, is discrete agency action, as we have discussed
above. General deficiencies in compliance, unlike the failure to
issue a ruling that was discussed in Safeway Stores , lack
the specificity requisite for agency action.

															   The principal purpose of the APA limitations
we have discussed—and of the traditional limitations upon mandamus
from which they were derived—is to protect agencies from undue
judicial interference with their lawful discretion, and to avoid
judicial entanglement in abstract policy disagreements which courts
lack both expertise and information to resolve. If courts were
empowered to enter general orders compelling compliance with broad
statutory mandates, they would necessarily be empowered, as well,
to determine whether compliance was achieved—which would mean that
it would ultimately become the task of the supervising court,
rather than the agency, to work out compliance with the broad
statutory mandate, injecting the judge into day-to-day agency
management. To take just a few examples from federal resources
management, a plaintiff might allege that the Secretary had failed
to “manage wild free-roaming horses and burros in a manner that is
designed to achieve and maintain a thriving natural ecological
balance,” or to “manage the [New Orleans Jazz National]
[H]istorical [P]ark in such a manner as will preserve and
perpetuate knowledge and understanding of the history of jazz,” or
to “manage the [Steens Mountain] Cooperative Management and
Protection Area for the benefit of present and future generations.”
16 U. S. C. §§1333(a), 410bbb–2(a)(1), 460nnn–12(b). The
prospect of pervasive oversight by federal courts over the manner
and pace of agency compliance with such congressional directives is
not contemplated by the APA.

															B

															   SUWA’s second claim is that BLM
failed to comply with certain provisions in its land use plans,
thus contravening the requirement that “[t]he Secretary shall
manage the public lands … in accordance with the land use plans …
when they are available.” 43 U. S. C. §1732(a); see also 43
CFR §1610.5–3(a) (2003) (“All future resource management
authorizations and actions … and subsequent more detailed or
specific planning, shall conform to the approved plan”). The
relevant count in SUWA’s second amended complaint alleged that BLM
had violated a variety of commitments in its land use plans, but
over the course of the litigation these have been reduced to two,
one relating to the 1991 resource management plan for the San
Rafael area, and the other to various aspects of the 1990 ORV
implementation plan for the Henry Mountains area.

															   The actions contemplated by the
first of these alleged commitments (completion of a route
designation plan in the San Rafael area), and by one aspect of the
second (creation of “use supervision files” for designated areas in
the Henry Mountains area) have already been completed,[ Footnote 3 ] and these claims are therefore
moot. There remains the claim, with respect to the Henry Mountains
plan, that “in light of damage from ORVs in the Factory Butte
area,” a sub-area of Henry Mountains open to ORV use, “the [plan]
obligated BLM to conduct an intensive ORV monitoring program.”
Brief for SUWA 7–8. This claim is based upon the plan’s statement
that the Factory Butte area “will be monitored and closed if
warranted.” App. 140. SUWA does not contest BLM’s assertion in the
court below that informal monitoring has taken place for some
years, see Brief for Appellee Secretary of Interior et al. in
No. 01–4009 (CA10), p. 23, but it demands continuing implementation
of a monitoring program . By this it apparently means to
insist upon adherence to the plan’s general discussion of “Use
Supervision and Monitoring” in designated areas, App. 148–149,
which (in addition to calling for the use supervision files that
have already been created) provides that “[r]esource damage will be
documented and recommendations made for corrective action,”
“[m]onitoring in open areas will focus on determining damage which
may necessitate a change in designation,” and “emphasis on use
supervision will be placed on [limited and closed areas].” Id., at 149. SUWA acknowledges that a monitoring program
has recently been commenced . Brief for SUWA 12. In light,
however, of the continuing action that existence of a “program”
contemplates, and in light of BLM’s contention that the program
cannot be compelled under §706(1), this claim cannot be considered
moot.

															   The statutory directive that BLM manage “in
accordance with” land use plans, and the regulatory requirement
that authorizations and actions “conform to” those plans, prevent
BLM from taking actions inconsistent with the provisions of a land
use plan. Unless and until the plan is amended, such actions can be
set aside as contrary to law pursuant to 5 U. S. C. §706(2).
The claim presently under discussion, however, would have us go
further, and conclude that a statement in a plan that BLM “will”
take this, that, or the other action, is a binding commitment that
can be compelled under §706(1). In our view it is not—at least
absent clear indication of binding commitment in the terms of the
plan.

															   FLPMA describes land use plans as tools by
which “present and future use is projected .” 43 U.
S. C. §1701(a)(2) (emphasis added). The implementing
regulations make clear that land use plans are a preliminary step
in the overall process of managing public lands— “designed to guide
and control future management actions and the development of
subsequent, more detailed and limited scope plans for resources and
uses.” 43 CFR §1601.0–2 (2003). The statute and regulations confirm
that a land use plan is not ordinarily the medium for affirmative
decisions that implement the agency’s “project[ions].”[ Footnote 4 ] Title 43 U. S. C. §1712(e)
provides that “[t]he Secretary may issue management decisions to
implement land use plans”—the decisions, that is, are distinct from
the plan itself. Picking up the same theme, the regulation defining
a land use plan declares that a plan “is not a final implementation
decision on actions which require further specific plans, process
steps, or decisions under specific provisions of law and
regulations.” 43 CFR §1601.0–5(k) (2003). The BLM’s Land Use
Planning Handbook specifies that land use plans are normally not
used to make site-specific implementation decisions. See Handbook
II–2.

															   Plans also receive a different agency review
process from implementation decisions. Appeal to the Department’s
Board of Land Appeals is available for “a specific action being
proposed to implement some portion of a resource management plan or
amendment.” 43 CFR §1610.5–3(b). However, the Board, which reviews
“decisions rendered by Departmental officials relating to … [t]he
use and disposition of public lands and their resources,”
§4.1(b)(3)(i), does not review the approval of a plan, since it
regards a plan as a policy determination, not an implementation
decision. See, e.g., Wilderness Society , 109
I. B. L. A. 175, 178 (1989); Wilderness
Society , 90 I. B. L. A. 221, 224 (1986); see
also Handbook II–2, IV–3. Plans are protested to the BLM director,
not appealed.

															   The San Rafael plan provides an apt
illustration of the immense scope of projected activity that a land
use plan can embrace. Over 100 pages in length, it presents a
comprehensive management framework for 1.5 million acres of
BLM-administered land. Twenty categories of resource management are
separately discussed, including mineral extraction, wilderness
protection, livestock grazing, preservation of cultural resources,
and recreation. The plan lays out an ambitious agenda for the
preparation of additional, more detailed plans and specific next
steps for implementation. Its introduction notes that “[a]n [ORV]
implementation plan is scheduled to be prepared within 1 year
following approval of the [San Rafael plan].” San Rafael Plan 9.
Similarly “scheduled for preparation” are activity plans for
certain environmentally sensitive areas, “along with allotment
management plans, habitat management plans, a fire management plan,
recreation management plans …, cultural resource management plans
for selected sites, watershed activity plans, and the wild and
scenic river management plan.” Ibid . The projected
schedule set forth in the plan shows “[a]nticipated
[i]mplementation” of some future plans within one year, others
within three years, and still others, such as certain recreation
and cultural resource management plans, at a pace of “one study per
fiscal year.” Id. , at 95–102.

															   Quite unlike a specific statutory command
requiring an agency to promulgate regulations by a certain date, a
land use plan is generally a statement of priorities; it guides and
constrains actions, but does not (at least in the usual case)
prescribe them. It would be unreasonable to think that either
Congress or the agency intended otherwise, since land use plans
nationwide would commit the agency to actions far in the future,
for which funds have not yet been appropriated. Some plans make
explicit that implementation of their programmatic content is
subject to budgetary constraints. See Brief for Petitioners 42–43,
and n. 18 (quoting from such plans). While the Henry Mountains
plan does not contain such a specification, we think it must
reasonably be implied. A statement by BLM about what it plans to
do, at some point, provided it has the funds and there are not more
pressing priorities, cannot be plucked out of context and made a
basis for suit under §706(1).

															   Of course, an action called for in a plan may
be compelled when the plan merely reiterates duties the agency is
already obligated to perform, or perhaps when language in the plan
itself creates a commitment binding on the agency. But allowing
general enforcement of plan terms would lead to pervasive
interference with BLM’s own ordering of priorities. For example, a
judicial decree compelling immediate preparation of all of the
detailed plans called for in the San Rafael plan would divert BLM’s
energies from other projects throughout the country that are in
fact more pressing. And while such a decree might please the
environmental plaintiffs in the present case, it would ultimately
operate to the detriment of sound environmental management. Its
predictable consequence would be much vaguer plans from BLM in the
future—making coordination with other agencies more difficult, and
depriving the public of important information concerning the
agency’s long-range intentions.

															   We therefore hold that the Henry Mountains
plan’s statements to the effect that BLM will conduct “use
supervision and monitoring” in designated areas—like other “will
do” projections of agency action set forth in land use plans—are
not a legally binding commitment enforceable under §706(1). That
being so, we find it unnecessary to consider whether the action
envisioned by the statements is sufficiently discrete to be
amenable to compulsion under the APA.[ Footnote 5 ]

															IV

															   Finally, we turn to SUWA’s
contention that BLM failed to fulfill certain obligations under
NEPA. Before addressing whether a NEPA-required duty is actionable
under the APA, we must decide whether NEPA creates an obligation in
the first place. NEPA requires a federal agency to prepare an
environmental impact statement (EIS) as part of any “proposals for
legislation and other major Federal actions significantly affecting
the quality of the human environment.” 42 U. S. C.
§4332(2)(C). Often an initial EIS is sufficient, but in certain
circumstances an EIS must be supplemented. See Marsh v. Oregon Natural Resources Council, 490 U. S. 360 , 370–374
(1989). A regulation of the Council on Environmental Quality
requires supplementation where “[t]here are significant new
circumstances or information relevant to environmental concerns and
bearing on the proposed action or its impacts.” 40 CFR
§1502.9(c)(1)(ii) (2003). In Marsh , we interpreted §4332
in light of this regulation to require an agency to take a “hard
look” at the new information to assess whether supplementation
might be necessary. 490 U. S., at 385; see id. , at
378–385.

															   SUWA argues that evidence of
increased ORV use is “significant new circumstances or information”
that requires a “hard look.” We disagree. As we noted in Marsh , supplementation is necessary only if “there remains
‘major Federal actio[n]’ to occur,” as that term is used in
§4332(2)(C). 490 U. S., at 374. In Marsh , that condition
was met: the dam construction project that gave rise to
environmental review was not yet completed. Here, by contrast,
although the “ [a]pproval of a [land use plan]” is a “major
Federal action” requiring an EIS, 43 CFR §1601.0–6 (2003) (emphasis
added), that action is completed when the plan is approved. The
land use plan is the “proposed action” contemplated by the
regulation. There is no ongoing “major Federal action” that could
require supplementation (though BLM is required to perform
additional NEPA analyses if a plan is amended or revised, see
§§1610.5–5, 5–6).

															* * *

															   The judgment of the Court of
Appeals is reversed, and the case is remanded for further
proceedings consistent with this opinion.

															It is so ordered. Footnote 1 Of course §706(1) also authorizes courts to
“compel agency action … unreasonably delayed”—but a delay cannot be
unreasonable with respect to action that is not required. Footnote 2 Title 5 U. S. C. §706(2) provides, in
relevant part:

															“The reviewing court shall— …

															   “(2) hold unlawful and set aside agency action
… found to be—

															   “(A) arbitrary, capricious, an abuse of
discretion, or otherwise not in accordance with
law … .” Footnote 3 See U. S. Dept. of Interior, BLM, San Rafael
Route Designation Plan (2003),
http://www.ut.blm.gov/sanrafaelohv/wtheplan.htm; 3 App. to Brief
for Appellants in No. 01–4009 (CA10), p. 771 (declaration of
manager for relevant BLM field office, noting the establishment of
monitoring files for the Henry Mountains area); Brief for SUWA 12
(acknowledging completion of these actions).

															   It is arguable that the complaint sought not
merely creation but continuing maintenance of use supervision
files, in which case (for the reasons set forth with respect to the
ORV monitoring program later in text) that claim would not be moot.
If so, what we say below with regard to the merits of the ORV
monitoring claim would apply equally to the use supervision file
claim. Footnote 4 The exceptions “are normally limited to those
required by regulation, such as designating [ORV] areas, roads, and
trails (see 43 CFR 8342).” U. S. Dept. of Interior, BLM, Land Use
Planning Handbook II–2 (2000) (hereinafter Handbook). See, e.g., U. S. Dept. of Interior, BLM, San Rafael Final
Resource Management Plan 63 (1991) (hereinafter San Rafael Plan)
(available at http://www.ut.blm.gov/planning/
OTHERS/SRARMP-ROD%20MAY%201991.pdf). Footnote 5 We express no view as to whether a court
could, under §706(1), enforce a duty to monitor ORV use imposed by
a BLM regulation, see 43 CFR §8342.3 (2003). That question is not
before us.

Summary: In Norton v. Southern Utah Wilderness Alliance (2004), the Supreme Court ruled that the Bureau of Land Management (BLM) was not required to take action to address off-road vehicle (ORV) use on public lands in Utah. The Court found that while the BLM had a duty to manage and protect these lands, the specific actions requested by the Southern Utah Wilderness Alliance were not required by the applicable statutes and regulations. The Court also held that the BLM's approval of a land-use plan was a "major federal action" under the National Environmental Policy Act (NEPA), but that supplementation of the plan was not necessary unless there were significant new circumstances or ongoing major federal actions.


OCTOBER TERM, 2001

															Syllabus

															BARNHART, COMMISSIONER OF SOCIAL SECURITY v. WALTON

															CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR THE FOURTH
CIRCUIT

															No. 00-1937. Argued January 16, 2002-Decided March 27, 2002

															The Social Security Act authorizes payment of Title II
disability insurance benefits and Title XVI Supplemental Security
Income to individuals who have an "inability to engage in
any substantial gainful activity by reason of any medically
determinable ... impairment ... which has lasted or can be
expected to last for a continuous period of not less than 12 months." 42 U. S. C. §423(d)(1)(A) (emphasis added); accord,
§ 1382c(a)(3)(A). The Social Security Administration (Agency)
denied benefits to respondent Walton, finding that his "inability"
to engage in substantial gainful activity lasted only 11 months.
The District Court affirmed, but the Fourth Circuit reversed,
holding that the 12-month duration requirement modifies
"impairment" not "inability," that the statute leaves no doubt that
no similar duration requirement relates to an "inability," and that
therefore Walton was entitled to benefits despite Agency
regulations restricting them to those unable to work for 12 months.
The court decided further that Walton qualified for benefits
because, prior to his return to work, his "inability" would have
been "expected" to last 12 months. It conceded that the Agency had
made Walton's actual return to work within 12 months of his onset
date and before the Agency's decision date determinative on this
point, 20 CFR §§ 404. 1520(b), 1592(d)(2), but found that the
regulations conflicted with the statute. It noted that Walton's
work simply counted as part of a 9-month trial work period during
which persons "entitled" to Title II benefits may work without loss
of benefits, 42 U. S. C. § 422(c). Held: The Agency's interpretations of the statute fall
within its lawful interpretative authority. pp. 217-225.

															(a) The Agency's reading of the term "inability" is reasonable.
The statute requires both an "inability" to engage in any
substantial gainful activity and an "impairment" providing "reason"
for the "inability," adding that the "impairment" must last or be
expected to last not less than 12 months. The Agency has determined
in both its formal regulations and its interpretation of those
regulations that the "inability" must last the same amount of time.
Courts grant considerable leeway to an agency's interpretation of
its own regulations, and the Agency has properly interpreted its
regulation here. Thus, this Court must decide 213 (1) whether the statute unambiguously forbids that
interpretation, and if not, (2) whether the interpretation exceeds
permissible bounds. Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837, 843.
First, the Act does not unambiguously forbid the regulation. That
the statute's 12-month phrase modifies only "impairment" shows only
that the provision says nothing explicitly about the "inability's"
duration. Such silence normally creates, but does not resolve,
ambiguity. Second, the Agency's construction is permissible. It
supplies a duration requirement, which the statute demands, in a
way that consistently reconciles the statutory "impairment" and
"inability" language. The Agency's regulations also reflect the
Agency's own longstanding interpretation, which should be accorded
particular deference, North Haven Bd. of Ed. v. Bell, 456 U. S. 512 ,
522, n. 12. Finally, Congress has frequently amended or reenacted
the relevant provisions without change. Walton's claim that Title
II's 5-month waiting period for entitlement protects against a
claimant with a chronic, but only briefly disabling, disease shows,
at most, that the Agency could have chosen other reasonable time
periods. Moreover, Title XVI has no such period, yet Walton offers
no explanation why its identical definitional language should be
interpreted differently in a closely related context. Walton's
argument that the Agency's interpretation should be disregarded
because its formal regulations were only recently enacted is also
rejected. E. g., Smiley v. Citibank (South Dakota), N.
A., 517 U. S.
735 , 741. And the Agency's longstanding interpretation is not
automatically deprived of the judicial deference otherwise its due
because it was previously reached through means less formal than
notice-and-comment rulemaking. Chevron, supra, at 843. Pp.
217-222.

															(b) Also consistent with the statute is the Agency's regulation
providing that "[y]ou are not entitled to a trial work
period" if "you perform work ... within 12 months of the onset of
the impairment ... and before the date of any ...
decision finding ... you ... disabled," 20 CFR § 404.1592(d)(2)
(emphasis added). The statute is ambiguous, and the regulation
treats a pre-Agency-decision actual return to work as if it were
determinative of the "can be expected to last" question. The
statute's complexity, the vast number of claims it engenders, and
the consequent need for agency expertise and administrative
experience lead the Court to read the statute as delegating to the
Agency considerable authority to fill in matters of detail related
to its administration. See Schweiker v. Gray
Panthers, 453 U.
S. 34 , 43-44. The interpretation at issue is such a matter. pp.
222-225. 235 F.3d
184 , reversed. 214 BREYER, J., delivered the opinion of the Court, Parts I and III
of which were unanimous, and Part II of which was joined by
REHNQUIST, C. J., and STEVENS, O'CONNOR, KENNEDY, SOUTER, THOMAS,
and GINSBURG, JJ. SCALIA, J., filed an opinion concurring in part
and concurring in the judgment, post, p. 226.

															Jeffrey A. Lamken argued the cause for petitioner. With him on
the briefs were Solicitor General Olson, Assistant Attorney General
McCallum, Deputy Solicitor General Kneedler, John C. Hoyle, and
Mark S. Davies. Kathryn L. Pryor argued the cause for respondent. With
her on the brief was James W Speer. * JUSTICE BREYER delivered the opinion of the Court.

															The Social Security Act authorizes payment of disability
insurance benefits and Supplemental Security Income to individuals
with disabilities. See 49 Stat. 622, as amended, 42 U. S. C. § 401 et seq. (1994 ed. and Supp. V) (Title II disability
insurance benefits); § 1381 et seq. (Title XVI supplemental
security income). For both types of benefits the Act defines the
key term "disability" as an "inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last for a continuous
period of not less than 12 months." § 423(d)(1)(A) (1994
ed.) (Title II) (emphasis added); accord, § 1382c(a)(3)(A) (1994
ed., Supp. V) (Title XVI). This case presents two questions about the Social Security
Administration's interpretation of this definition.

															First, the Social Security Administration (which we shall call
the Agency) reads the term "inability" as including a "12 month"
requirement. In its view, the "inability" (to engage in any
substantial gainful activity) must last, or must be ex- * Rochelle Bobroff, Michael Schuster, and Robert E. Rains
filed a brief for AARP et al. as amici curiae urging
affirmance. 215 pected to last, for at least 12 months. Second,
the Agency reads the term "expected to last" as applicable only
when the "inability" has not yet lasted 12 months. In the
case of a later Agency determination-where the "inability" did
not last 12 months-the Agency will automatically assume that
the claimant failed to meet the duration requirement. It will not
look back to decide hypothetically whether, despite the claimant's
actual return to work before 12 months expired, the "inability"
nonetheless might have been expected to last that long.

															The Court of Appeals for the Fourth Circuit held both these
interpretations of the statute unlawful. We hold, to the contrary,
that both fall within the Agency's lawful interpretive authority.
See Chevron U. S. A. Inc. v. Natural Resources
Defense Council, Inc., 467 U. S. 837 (1984).
Consequently, we reverse.

															I

															In 1996 Cleveland Walton, the respondent, applied for both Title
II disability insurance benefits and Title XVI Supplemental
Security Income. The Agency found that (1) by October 31, 1994,
Walton had developed a serious mental illness involving both
schizophrenia and associated depression; (2) the illness caused him
then to lose his job as a full-time teacher; (3) by mid-1995 he
began to work again part time as a cashier; and (4) by December
1995 he was working as a cashier full time.

															The Agency concluded that Walton's mental illness had prevented
him from engaging in any significant work, i. e., from
"engag[ing] in any substantial gainful activity," for 11
months-from October 31, 1994 (when he lost his teaching job) until
the end of September 1995 (when he earned income sufficient to rise
to the level of "substantial gainful activity"). See 20 CFR §§
404.1574, 416.974 (2001). And because the statute demanded an
"inability to engage in any substantial gainful activity" lasting
12, not 11, months, Walton was not entitled to benefits. 216 Walton sought court review. The District Court affirmed the
Agency's decision, but the Court of Appeals for the Fourth Circuit
reversed. Walton v. Apfel, 235
F.3d 184 , 186-187 (2000). The court said that the statute's
12-month duration requirement modifies the word "impairment," not
the word "inability." Id., at 189. It added that the
statute's "language ... leaves no doubt" that there is no similar
"duration requirement" related to an "inability" (to engage in
substantial gainful activity). Ibid. It concluded that,
because the statute's language "speaks clearly" and is
"unambiguous," Walton was entitled to receive benefits despite
agency regulations restricting benefits to those unable to work for
a 12-month period. Ibid. The court went on to decide that, in any event, Walton qualified
because, prior to Walton's return to work, one would have
"expected" his "inability" to last 12 months. Id., at
189-190. It conceded that the Agency had made Walton's actual
return to work determinative on this point. See 20 CFR §§
404.1520(b), 1592(d)(2) (2001). But it found unlawful the Agency
regulations that gave the Agency the benefit of hindsight-on the
ground that they conflicted with the statute's clear command. 235
F. 3d, at 190.

															For either reason, the Fourth Circuit concluded, Walton became
"entitled" to Title II benefits no later than April 1995, five
months after the onset of his illness. See 42 U. S. C. §§
423(a)(1)(D)(i), 423(a)(1)(D)(ii) (providing for a 5-month "waiting
period" before a claimant is "entitled" to benefits), 423(c)(2)(A)
(1994 ed.). It added that Walton's later work as a cashier was
legally beside the point. That work simply counted as part of a
9-month "trial work period," which the statute grants to those
"entitled" to Title II benefits, and which it permits them to
perform without loss of benefits. § 422(c).

															The Government sought certiorari. It pointed out that the Fourth
Circuit's first holding conflicts with those of other Circuits,
compare 235 F. 3d, at 189-190, with Titus v. Sulli- 217 van, 4 F.3d
590 , 594-595 (CA8 1993), and Alexander v. Richardson, 451 F.2d
1185 (CAlO 1971). It added that the Fourth Circuit's views were
contrary to well-settled law and would create additional Social
Security costs of $80 billion over 10 years. We granted the writ.
We now reverse.

															II

															The statutory definition of "disability" has two parts.

															First, it requires a certain kind of "inability," namely, an
"inability to engage in any substantial gainful activity." Second,
it requires an "impairment," namely, a "physical or mental
impairment," which provides "reason" for the "inability." The
statute adds that the "impairment" must be one that "has lasted or
can be expected to last ... not less than 12 months." But what
about the "inability"? Must it also last (or be expected to last)
for the same amount of time?

															The Agency has answered this question in the affirmative.

															Acting pursuant to statutory rulemaking authority, 42 U. S. C.
§§ 405(a) (Title II), 1383(d)(1) (Title XVI), it has promulgated
formal regulations that state that a claimant is not disabled
"regardless of [his] medical condition," if he is doing
"substantial gainful activity." 20 CFR § 404.1520(b) (2001). And
the Agency has interpreted this regulation to mean that the
claimant is not disabled if "within 12 months after the onset of an
impairment ... the impairment no longer prevents substantial
gainful activity." 65 Fed. Reg. 42774 (2000). Courts grant an
agency's interpretation of its own regulations considerable legal
leeway. Auer v. Robbins, 519 U. S. 452, 461 (1997); Udall v. Tallman, 380 U. S. 1 , 16-17 (1965).
And no one here denies that the Agency has properly interpreted its
own regulation.

															Consequently, the legal question before us is whether the
Agency's interpretation of the statute is lawful. This Court has
previously said that, if the statute speaks clearly "to the precise
question at issue," we "must give effect to the unambiguously
expressed intent of Congress." Chevron, 467 218 u. S., at 842-843. If, however, the statute "is silent or
ambiguous with respect to the specific issue," we must sustain the
Agency's interpretation if it is "based on a permissible
construction" of the Act. Id., at 843. Hence we must decide
(1) whether the statute unambiguously forbids the Agency's
interpretation, and, if not, (2) whether the interpretation, for
other reasons, exceeds the bounds of the permissible. Ibid.; see also United States v. Mead Corp., 533 U. S. 218 , 227
(2001).

															First, the statute does not unambiguously forbid the regulation.
The Fourth Circuit believed the contrary primarily for a linguistic
reason. It pointed out that, linguistically speaking, the statute's
"12-month" phrase modifies only the word "impairment," not the word
"inability." And to that extent we agree. After all, the statute,
in parallel phrasing, uses the words "which can be expected to
result in death." And that structurally parallel phrase makes sense
in reference to an "impairment," but makes no sense in reference to
the "inability."

															Nonetheless, this linguistic point is insufficient. It shows
that the particular statutory provision says nothing explicitly
about the "inability's" duration. But such silence, after all,
normally creates ambiguity. It does not resolve it.

															Moreover, a nearby provision of the statute says that an "individual shall be determined to be under a disability only if
his ... impairment ... [is] of such severity that he is not only
unable to do his previous work but cannot ... engage in any other
kind of substantial gainful work which exists in the national
economy." 42 U. S. C. § 423(d)(2)(A) (Title II); accord, §
1382c(a)(3)(B) (Title XVI). In other words, the statute, in the two provisions, specifies
that the "impairment" must last 12 months and also be severe enough
to prevent the claimant from engaging in virtually any "substantial
gainful work." The statute, we con- 219 cede, nowhere explicitly says that the "impairment" must be that severe (i. e., severe enough to prevent
"substantial gainful work") for 12 months. But that is a fair
inference from the language. See Brief for AARP et al. as Amici
Curiae 13 (conceding that an impairment must remain of
"disabling severity" for 12 months). At the very least the statute
is ambiguous in that respect. And, if so, then it is an equally
fair inference that the "inability" must last 12 months. That is
because the latter statement (i. e., that the claimant must
be unable to "engage in any substantial gainful activity" for a
year) is the virtual equivalent of the former statement (i. e., that the "impairment" must remain severe enough to
prevent the claimant from engaging in "substantial gainful work"
for a year). It simply rephrases the same point in a slightly
different way.

															Second, the Agency's construction is "permissible." The
interpretation makes considerable sense in terms of the statute's
basic objectives. The statute demands some duration requirement. No
one claims that the statute would permit an individual with a
chronic illness-say, high blood pressure-to qualify for benefits if
that illness, while itself lasting for a year, were to permit a
claimant to return to work after only a week, or perhaps even a
day, away from the job. The Agency's interpretation supplies a
duration requirement, which the statute demands, while doing so in
a way that consistently reconciles the statutory "impairment" and
"inability" language.

															In addition, the Agency's regulations reflect the Agency's own
longstanding interpretation. See Social Security Ruling 82-52, p.
106 (cum. ed. 1982) ("In considering 'duration,' it is the
inability to engage in [substantial gainful activity] that must
last the required 12-month period"); Disability Insurance State
Manual § 316 (Sept. 9, 1965), Government Lodging, Tab C, § 316
("Duration of impairment refers to that period of time during which
an individual is continuously unable to engage in substantial
gainful activity because 220 of" an impairment); OASI Disability Insurance Letter No. 39
(Jan. 22, 1957), id., Tab A, p. 1 (duration requirement
refers to the "expected duration of the medical impairment" at a "level of severity sufficient to preclude" substantial gainful
activity"). And this Court will normally accord particular
deference to an agency interpretation of "longstanding" duration. North Haven Bd. of Ed. v. Bell, 456 U. S. 512 , 522,

															Finally, Congress has frequently amended or reenacted the
relevant provisions without change. E. g., Social Security
Amendments of 1965, § 303(a)(1), 79 Stat. 366; see also S. Rep. No.
404, 89th Cong., 1st Sess., pt. I, pp. 98-99 (1965) ("[T]he
committee's bill ... provide[s] for the payment of disability
benefits for an insured worker who has been or can be expected to
be totally disabled throughout a continuous period of 12
calendar months" (emphasis added)); id., at 98 (rejecting
effort to provide benefits to those with "shortterm, temporary
disabilit[ies]," defined as inability to work for six months); H.
R. Rep. No. 92-231, p. 56 (1971) ("No benefit is payable, however,
unless the disability is expected to last (or has lasted) at
least 12 consecutive months" (emphasis added)); S. Rep. No. 744,
90th Cong., 1st Sess., 49 (1967) ("The committee also believes ...
that an individual who does substantial gainful work despite an
impairment or impairments that otherwise might be considered
disabling is not disabled for purposes of establishing a period of
disability"). These circumstances provide further evidence-if more
is needed-that Congress intended the Agency's interpretation, or at
least understood the interpretation as statutorily permissible. Commodity Futures Trading Comm'n Walton points in reply to Title II language stating that a
claimant who is "under a disability ... shall be entitled to a ...
benefit ... beginning with the first month after" a "waiting
period" of "five consecutive calendar months ... throughout which"
he "has been under a disa- 221 bility." 42 U. S. C. §§ 423(a)(1)(D)(i), 423(c)(2)(A). He adds
that this 5-month "waiting period" assures a lengthy period of time
during which the applicant (who must be "under a disability"
throughout) has been unable to work. And it thereby provides
ironclad protection against the claimant who suffers a chronic, but
only briefly disabling, disease, such as the claimant who suffers
high blood pressure in our earlier example. See supra, at
219. This claim does not help Walton, however, for it shows, at
most, that the Agency might have chosen other reasonable time
periods-a matter not disputed. Regardless, Walton's "waiting
period" argument could work only in respect to Title II, not Title
XVI. Title XVI has no waiting period, though it uses identical
definitional language. And Walton does not explain why we should
interpret the same statutory words differently in closely related
contexts. See Department of Revenue of Ore. v. ACF
Industries, Inc., 510 U. S. 332 , 342 (1994)
(" '[I]dentical words used in different parts of the same act are
intended to have the same meaning'" (quoting Sorenson v. Secretary of Treasury, 475 U. S. 851 , 860 (1986)
(some internal quotation marks omitted)).

															Walton also asks us to disregard the Agency's interpretation of
its formal regulations on the ground that the Agency only recently
enacted those regulations, perhaps in response to this litigation.
We have previously rejected similar arguments. Smiley v. Citibank (South Dakota), N. A., 517 U. S. 735 , 741
(1996); United States v. Morton, 467 U. S. 822 , 835836, n.
21 (1984).

															Regardless, the Agency's interpretation is one of long standing.
See supra, at 220. And the fact that the Agency previously
reached its interpretation through means less formal than "notice
and comment" rulemaking, see 5 U. S. C. § 553, does not
automatically deprive that interpretation of the judicial deference
otherwise its due. Cf. Chevron, 467 U. S., at 843 (stating,
without delineation of means, that the "'power of an administrative
agency to administer a congres- 222 sionally created ... program necessarily requires the
formulation of policy'" (quoting Morton v. Ruiz, 415 U. S. 199 ,
231 (1974))). If this Court's opinion in Christensen v. Harris County, 529 U. S. 576 (2000),
suggested an absolute rule to the contrary, our later opinion in United States v. Mead Corp., 533 U. S. 218 (2001),
denied the suggestion. Id., at 230-231 ("[T]he want of"
notice and comment "does not decide the case"). Indeed, Mead pointed to instances in which the Court has applied Chevron deference to agency interpretations that did not emerge out of
notice-and-comment rulemaking. 533 U. S., at 230-231 (citing NationsBank of N. c., N. A. v. Variable Annuity
Life Ins. Co., 513 U. S. 251 , 256257
(1995)). It indicated that whether a court should give such
deference depends in significant part upon the interpretive method
used and the nature of the question at issue. 533 U. S., at
229-231. And it discussed at length why Chevron did not
require deference in the circumstances there present-a discussion
that would have been superfluous had the presence or absence of
notice-and-comment rulemaking been dispositive. 533 U. S., at
231-234.

															In this case, the interstitial nature of the legal question, the
related expertise of the Agency, the importance of the question to
administration of the statute, the complexity of that
administration, and the careful consideration the Agency has given
the question over a long period of time all indicate that Chevron provides the appropriate legal lens through which to
view the legality of the Agency interpretation here at issue. See United States v. Mead Corp., supra; cf. also 1 K.
Davis & R. Pierce, Administrative Law Treatise §§ 1.7,3.3 (3d
ed. 1994).

															For these reasons, we find the Agency's interpretation
lawful.

															III

															Walton's second claim is more complex. For purposes of making
that claim, Walton assumes what we have just decided, namely, that
the statute's "12 month" duration require- 223 ments apply to both the "impairment" and the "inability" to work
requirements. Walton also concedes that he returned to work after
11 months. But Walton claims that his work from month 11 to month
12 does not count against him because it is part of a "trial work"
period that the statute grants to those "entitled" to Title II
benefits. See 42 U. S. C. § 422(c). And Walton adds, he was
"entitle[d]" to benefits because-even though he returned to work
after 11 months-his "impairment" and his "inability" to work were
nonetheless "expected to last" for at least "12 months" before he returned to work.

															To illustrate Walton's argument, we simplify the actual
circumstances. We imagine: (1) On January 1, Year One, Walton
developed (a) a severe impairment, which (b) made him unable to
work; (2) Eleven (not twelve) months later, on December 1, Year
One, Walton returned to work; (3) On July 1, Year Two, the Agency
adjudicated, and denied, Walton's claim for benefits. Walton argues
that, even though he returned to work after 11 months, had the
Agency looked at the matter, not ex post, but as if it were
looking prior to his return to work, the Agency would have
had to conclude that both his "impairment" and his "inability" to
work "can be expected to last for a continuous period of not
less than 12 months." § 423(d)(1)(A). He consequently satisfied the
12month duration requirement and became "entitled" to benefits
before he returned to work; he was in turn entitled to a "trial
work" period; and his subsequent work as a cashier, being "trial
work," should not count against him.

															The Agency's regulations plainly reject this view of the
statute. They say, "You are not entitled to a trial work
period" if "you perform work ... within 12 months of the onset of
the impairment(s) ... and before the date of any notice of determination or decision finding ... you ...
disabled." 20 CFR § 404. 1592(d)(2) (2001). This regulation
means that the Agency, deciding before the end of Year One, might
have found that Walton's impairment (or inability to work) "can 224 be expected to last" for 12 months. But the Agency,
deciding after Year One in which Walton in fact returned to work,
would not ask whether his impairment (or inability to work) could have been expected to last 12 months.

															The legal question is whether this Agency regulation is
consistent with the statute. The Court of Appeals, accepting
Walton's view, concluded that it is not. It said that the Agency's
rules-permitting the use of hindsight when reviewing claims-are
inconsistent with the statute's plain language, 235 F. 3d, at 191.
And, here, other courts have agreed. See Salamalekis v. Commissioner of Soc. Sec., 221 F.3d
828 (CA6 2000); Newton v. Chater, 92 F.3d
688 (CA8 1996); Walker v. Secretary of Health and
Human Servs., 943 F. 2d 1257 (CAlO 1991); McDonald v. Bowen, 818 F.2d
559 (CA71986). Nonetheless, we believe that Agency regulation is lawful.

															See Chevron, supra, at 843. The statute is ambiguous. It
says nothing about how the Agency, when it adjudicates a matter
after Year One, is to treat an earlier return to work. Its language
"can be expected to last" 12 months, 42 U. S. C. § 423(d)(1)(A),
simply does not say as of what time the law measures the
"expectation." Indeed, from a linguistic perspective, the phrase
"can be expected" foresees a decisionmaker who is looking into the
future, not a decisionmaker who is in the future, looking back into
the past in order to see what then "was," "could be," or "could
have been" expected. And read in context, the purpose of the phrase
"can be expected to last" might be one of permitting the Agency to
award benefits before 12 months have expired, not one of denying
the Agency the benefit of hindsight. See 65 Fed. Reg., at 42780;
cf. also S. Rep. No. 404, at 99.

															At the same time, the Agency's regulation seems a reasonable,
hence permissible, interpretation of the statute. In effect it
treats a pre-Agency-decision actual return to work, e. g., Walton's return in December Year One, as if it were
determinative of the expectation question. With Year Two's
hindsight, Walton's "inability" to work "can" not "be 225 expected to last 12 months." And use of that hindsight avoids
the need for the Year Two decisionmaker in effect to answer a
highly unwieldy question in what grammarians might call the
pluperfect future tense.

															Of course, administrators and judges are capable of answering
hypothetical questions of this kind. But here the question concerns
what must be a contrary-to-fact speculation about the future. It is
a speculation that, however often raised, would rarely prove easy
to resolve. And the statute's purpose does not demand its
resolution. Indeed, one might ask why, other things being equal, a
claimant who returns to work too early ordinarily to qualify for
benefits nonetheless should qualify if, but only if, that return
was a kind of medical surprise. Of course, as Walton says, such
a rule would help encourage (or at least not discourage) a
claimant's early return to work. See generally S. Rep. No. 1856,
86th Cong., 2d Sess., 15-16 (1960). But the statute does not demand
that the Agency make of this desirable end an overriding
interpretive principle. And the Agency has recognized and addressed
the problem of work disincentives in other ways. See, e. g., 20 CFR §§ 404.1574(c), 404.1575(d) (2001).

															The statute's complexity, the vast number of claims that it
engenders, and the consequent need for agency expertise and
administrative experience lead us to read the statute as delegating
to the Agency considerable authority to fill in, through
interpretation, matters of detail related to its administration.
See Schweiker v. Gray Panthers, 453 U. S. 34 , 43-44
(1981). The interpretation at issue here is such a matter. The
statute's language is ambiguous. And the Agency's interpretation is
reasonable.

															We conclude that the Agency's regulation is lawful.

															***

															The judgment of the Fourth Circuit is

															Reversed. 226 Opinion of SCALIA, J.

															JUSTICE SCALIA, concurring in part and concurring in the
judgment.

															I join all but Part II of the Court's opinion.

															I agree that deference is owed to regulations of the Social
Security Administration (SSA) interpreting the definition of
"disability," 42 U. S. C. §§ 423(d)(1)(A), 1382c(a)(3)(A) (1994 ed.
and Supp. V). See 65 Fed. Reg. 42774 (2000). As the Court
acknowledges, the recency of these regulations is irrelevant, see ante, at 220-221 (citing Smiley v. Citibank (South
Dakota), N. A., 517 U. S. 735 , 741
(1996); United States v. Morton, 467 U. S. 822 , 835-836,
n. 21 (1984)). I would therefore not go on, as the Court does, ante, at 219222, to address the SSA's prior interpretation
of the definition of "disability" in a 1982 Social Security Ruling,
a 1965 Disability Insurance State Manual, and a 1957 OASI
Disability Insurance Letter.

															I do not believe, to begin with, that "particular deference" is
owed "to an agency interpretation of 'longstanding' duration," ante, at 220. That notion is an anachronism-a relic of the pre-Chevron days, when there was thought to be only one
"correct" interpretation of a statutory text. A "longstanding"
agency interpretation, particularly one that dated back to the very
origins of the statute, was more likely to reflect the single
correct meaning. See, e. g., Watt v. Alaska, 451 U. S. 259 , 272-273
(1981). But once it is accepted, as it was in Chevron, that
there is a range of permissible interpretations, and that the
agency is free to move from one to another, so long as the most
recent interpretation is reasonable its antiquity should make no
difference. Cf. Rust v. Sullivan, 500 U. S. 173 , 186-187
(1991); Chevron U. S. A. Inc. v. Natural Resources
Defense Council, Inc., 467 U. S. 837, 863-864 (1984).

															If, however, the Court does wish to credit the SSA's earlier
interpretations-both for the purpose of giving the agency's
position "particular deference" and for the purpose of relying upon
congressional reenactment with presumed knowledge 227 of the agency position, see ante, at 219-220-then I think
the Court should state why those interpretations were authoritative
enough (or whatever-else-enough Mead requires) to qualify
for deference. See United States v. Mead Corp., 533 U. S. 218 (2001). I of course agree that more than
noticeand-comment rulemaking qualifies, see ante, at
221-222, but that concession alone does not validate the Social
Security Ruling, the Disability Insurance State Manual, and the
OASI Disability Insurance Letter. (Only the latter two, I might
point out, antedate the congressional reenactments upon which the
Court relies.)

															The SSA's recently enacted regulations emerged from
notice-and-comment rulemaking and merit deference. No more need be
said.

Summary:
The Social Security Administration (SSA) denied benefits to respondent Walton, who had been unable to work for 11 months due to a medical impairment. The Fourth Circuit reversed, holding that the SSA's interpretation of the Social Security Act, which requires a duration of 12 months of inability to work for benefits, conflicted with the statute. The Supreme Court reversed, finding that the Agency's interpretation falls within its lawful interpretative authority. The Court granted leeway to the Agency's interpretation of its regulations and found that the statute does not unambiguously forbid the Agency's interpretation.

Justice Scalia concurred in part and concurred in the judgment, agreeing that deference is owed to the SSA's regulations but disagreeing with the Court's reliance on the SSA's prior interpretations. He argued that the "longstanding" agency interpretation is irrelevant once it is accepted that there is a range of permissible interpretations and that the agency is free to move between them.