In this thesis I argue that we should break with the dominant Kripkean tradition concerning natural kind terms and theoretical identity. I claim that there is just no interesting connection between the metaphysics and semantics of natural kind terms, and demonstrate this by constructing a version of descriptivism that is combined with the same metaphysics – that is, a nontrivial version of essentialism – found in Kripke, but which effectively avoids all of the standard criticisms. With my version of descriptivism (...) in place, I present what I take to be the most reasonable version of metaphysical essentialism, positing only what I call 'thin' essences. I claim that thin essences are perfectly adequate to underpin scientific realism, and moreover that they are sufficient to support the version of descriptivism developed here. In effect, what I offer here is an error theory of the Kripkean tradition: Kripke is right to think that there are interesting things to say about meaning and essence, but just wrong about what those things are. Thus whilst Kripke thinks that it is possible to make discoveries about the meanings of natural kind terms, I think, rather, that we make empirical discoveries that lead to revisions in meaning. Furthermore, whilst Kripke thinks there is a dichotomy between de re and de dicto necessity, and that theoretical identities are necessary de re, I think this distinction is both misleading and inaccurate, and that the necessity of theoretical identities is neither entirely de re nor entirely de dicto. By separating and insulating questions concerning meaning from questions concerning essence I show that whilst scientific discoveries are contingent and a posteriori, the definition of scientific terms are both necessary and a priori. (shrink)