  Searches for resonances decaying into pairs of jets are performed using
proton-proton collision data collected at $\sqrt{s} =$ 13 TeV corresponding to
an integrated luminosity of up to 36 fb$^{-1}$. A low-mass search, for
resonances with masses between 0.6 and 1.6 TeV, is performed based on events
with dijets reconstructed at the trigger level from calorimeter information. A
high-mass search, for resonances with masses above 1.6 TeV, is performed using
dijets reconstructed offline with a particle-flow algorithm. The dijet mass
spectrum is well described by a smooth parameterization and no evidence for the
production of new particles is observed. Upper limits at 95% confidence level
are reported on the production cross section for narrow resonances with masses
above 0.6 TeV. In the context of specific models, the limits exclude string
resonances with masses below 7.7 TeV, scalar diquarks below 7.2 TeV, axigluons
and colorons below 6.1 TeV, excited quarks below 6.0 TeV, color-octet scalars
below 3.4 TeV, W' bosons below 3.3 TeV, Z' bosons below 2.7 TeV,
Randall-Sundrum gravitons below 1.8 TeV and in the range 1.9 to 2.5 TeV, and
dark matter mediators below 2.6 TeV. The limits on both vector and axial-vector
mediators, in a simplified model of interactions between quarks and dark matter
particles, are presented as functions of dark matter particle mass and coupling
to quarks. Searches are also presented for broad resonances, including for the
first time spin-1 resonances with intrinsic widths as large as 30% of the
resonance mass. The broad resonance search improves and extends the exclusions
of a dark matter mediator to larger values of its mass and coupling to quarks.

High Energy Physics - Experiment