Naturalism about natural kinds is the view that they are none other than the kinds discoverable by science. This thesis is in tension with what is perhaps the dominant contemporary view of natural kinds: essentialism. According to essentialism, natural kinds constitute a small subset of our scientifi c categories, namely those defi nable in terms of intrinsic, microphysical properties, which are possessed necessarily rather than contingently by their bearers. Though essentialism may appear compatible with naturalism, and is indeed sometimes qualifi ed with the epithet “scientifi c,” it has become increasingly clear in recent years that only a minority of categories posited by science satisfy those conditions. If one does not limit oneself to basic physics and chemistry, the categories found in the biological and other special sciences often violate one or more of the conditions that essentialists impose upon natural kinds. Indeed, I would argue that even when one does limit oneself to the basic sciences, the strictures of essentialism do not apply to all their categories. However, I will not try to argue against essentialism directly in this chapter. Instead, I will attempt to articulate an alternative, naturalist conception of natural kinds, according to which the mark of natural kinds is their discoverability by science, not just basic science but the special sciences and even the social sciences. I will locate the origins of this naturalist conception in the work of John Stuart Mill, then I will trace it through the works of W. V. Quine, John Dupré, and Richard Boyd. In each case, I will defend some aspects of the views of these philosophers while taking issue with other aspects. What will emerge is a preliminary defense of a naturalist account of natural kinds, which should provide a contrast with the prevailing essentialist conception.