This paper discusses the Aristotelian notions of matter and form as they are treated in the philosophy of Leibniz. The discussion is divided into three parts, corresponding to three periods in Leibniz's development. In the earliest period, as exemplified in a 1669 letter to his former mentor Jakob Thomasius, Leibniz argues that matter and form can be given straightforward interpretations in terms of size and shape, basic categories in the new mechanical philosophy. In Leibniz's middle years, on the other hand, as exemplified in the Discourse on Metaphysics and the correspondence with Arnauld, Leibniz seems to hold a more orthodox Aristotelian view of matter and form as the constituents of the corporeal substances that ground the reality of the physical world. In Leibniz's latest years, as discussed in the letters with Des Bosses, matter and form enter once again in connection with the vinculum substantiale, the substantial bond that is supposed to bind monads together to form corporeal substances.