 A rigid designator designates the same object in all possible worlds in which that object exists and never designates anything else. This technical concept in the philosophy of language has critical consequences felt throughout philosophy. In their fullest generality, the consequences are metaphysical and epistemological. Whether a statement’s designators are rigid or non-rigid may determine whether it is necessarily true, necessarily false, or contingent. This metaphysical status is sometimes out of accord with what one would expect given a statement’s apparent epistemological status as a posteriori or a priori. Statements affected include central ones under investigation in philosophical subdisciplines from the philosophy of science to mind to ethics and aesthetics. Hence, much of the discussion in various subdisciplines of philosophy is explicitly or implicitly framed around the distinction between rigid and non-rigid designators.