 Hylomorphism thus finds a range of applications across Aristotle’s corpus. This entry focuses on its genesis and development in the Physics and Metaphysics, in order to characterize and assess its fundamental features and core commitments. There is in any case already a considerable controversy at this basic level about what Aristotle means by matter and form: what precisely they are, how they are related to one another, how Aristotle intends to marshal arguments in support of them, and how best to deal with reasonable objections to their metaphysical consequences. We will begin by examining how Aristotle introduces his twin notions. Then we will move on to discuss some of the most important interpretative controversies: does Aristotle believe in so-called “prime” matter? Does matter or form serve as the principle of individuation in his metaphysics? Do natural forms include a specification of the kind of matter that anything of that form has to have?