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 ref