 Much recent interest in natural kinds has been stimulated by debates concerning the semantics of natural kind terms, originating especially with the work of Saul Kripke (1972) and Hilary Putnam (1973, 1975a). Typical natural kind expressions fall into one of two categories: predicates (is water, is gold, is a tiger, and is an oak) and singular terms (water, gold, the tiger, and the oak). In this section we survey some of the debates concerning the meaning of such terms, what—if anything—they refer to, and how they do so. Many discussions focus on the predicate kind terms, often considering them alongside other general terms, taking the principal issue to be the relationship between a natural kind term and its extension. Such an approach will appeal to conventionalists, but it may appeal also to those naturalists who are nominalists about natural kinds or otherwise hostile or indifferent to (strong) realism about natural kinds. However, we shall see that there may be reason to take the singular kind terms as primary. (One difference between Putnam and Kripke is that while Putnam tends to focus on the natural kind predicates, Kripke’s arguments are centered primarily on the singular natural kind terms.) Kripke and Putnam attacked a pair of related views about natural kind terms: descriptivist semantics (the meaning of such a term is a description satisfied by all and only members of the kind) and internalism (the meaning of such a term, the relevant description, is fully grasped by competent users of the term so that what fixes the extension of the term supervenes on what is internal to the user). Putnam’s rejection of descriptivism is a component of his argument that ‘meanings ain’t in the head’ whereas for Kripke the goal is to show that natural kind terms are much like proper names, for which, he argues, a descriptivist semantics fails. Members of the extension of a natural kind share properties in common. If descriptivism were true, then one would reasonably believe that those properties would be the properties mentioned in the description with the possible addition of further properties nomically correlated with those. But if descriptivism is false, it would appear that the characteristic properties of a kind will not typically be knowable a priori, but must be uncovered by scientific investigation. If so, then those properties, being necessary conditions of kind membership but discoverable only a posteriori, look very much like real essences. At first sight it seems then that the semantic arguments of Kripke and Putnam have metaphysical consequences. This is highly controversial (Salmon 1979, 1982). Do the arguments of Putnam and Kripke really support an essentialist metaphysics? Or do they illegitimately assume it? Or do they, more innocently, show that we as users of natural kind terms implicitly assume such a metaphysics?