Essentialism--roughly, the view that natural kinds have discrete essences, generating truths that are necessary but knowable only a posteriori--is an increasingly popular view in the metaphysics of science. At the same time, philosophers of language have been subjecting Kripkes views about the existence and scope of the necessary a posteriori to rigorous analysis and criticism. Essentialists typically appeal to Kripkean semantics to motivate their radical extension of the realm of the necessary a posteriori; but they rarely attempt to provide any semantic arguments for this extension, or engage with the critical work being done by philosophers of language. This collection brings authors on both sides together in one volume, thus helping the reader to see the connections between views in philosophy of language on the one hand and the metaphysics of science on the other. The result is a book that will have a significant impact on the debate about essentialism, encouraging essentialists to engage with debates about the semantic presuppositions that underpin their position, and, encouraging philosophers of language to engage with the metaphysical presuppositions enshrined in Kripkean semantics.