The author's concept of incommensurability is explicated by elaborating the claim that some terms essential to the formulation of older theories defy translation into the language of more recent ones. Defense of this claim rests on the distinction between interpreting a theory in a later language and translating the theory into it. The former is both possible and essential, the latter neither. The interpretation/translation distinction is then applied to Kitcher's critique of incommensurability and Quine's conception of a translation manual, both (...) of which take reference-preservation as the sole semantic criterion of translational adequacy. The paper concludes by enquiring about the additional criteria a successful translation must satisfy. (shrink)