Aquinas has much to say about individuation over the course of his career. Although certain aspects of his views appear to undergo development, there is one aspect that remains constant throughout—namely, his commitment to assigning both prime matter and quantity an essential role in the individuation of substances. This paper examines the vexed issue of how either prime matter or quantity, as Aquinas understands them, could have any role to play in this context. In the course of doing so, the (...) author attempts to put to rest a number of longstanding worries about the coherence of Aquinas’s views about individuation, as well as to draw out some of his broader commitments in metaphysics and natural philosophy that have yet to be fully appreciated. (shrink)