There are certainly some scientific categories which are merely conveniences introduced to organize complicated data, but others correspond to genuine features of the world. These are indispensable for successful science in some domain; in short, they are natural kinds. This book gives a general account of what it is to be a natural kind. It untangles philosophical puzzles surrounding natural kinds. Natural kinds can be practical, and we can identify them without pretending to know the fundamental structure of reality. The account is then put to work to illuminate specific examples, such as the category planet and the fate of Pluto, species like the common mallard and the species category itself, cognition and distributed cognition, animal signals and the threats they signify, and even baked goods.