There have been many recent books on Aristotle's theory of substance. This one is distinct from previous efforts in several ways. First, it offers a completely new and coherent interpretation of Aristotle's claim that substances are separate: substances turn out to be specimens of natural kinds. Second, it covers a broad range of issues, including Aristotle's criticism of Plato, his views on numerical sameness and identity, his epistemology, and his account of teleology. It also includes a discussion of much of the recent literature on Aristotle.