This paper considers the living body as matter in the De Anima, menstrual fluid as matter in the Generation and Corruption, and the prime matter of the elements in Generation and Corruption. I argue that there is a single, coherent notion of matter found in all three treatises, and at the same time important differences that are a result of (1) the different sorts of changes under consideration, and (2) whether Aristotle is considering the matter for a specific change, or the matter for all of a substance's characteristic changes. The single coherent notion is this: matter, by virtue of itself, is what does the changing, in the passive sense of what undergoes a given change. The question then is what is the thing that is properly suited to undergo the changes in question. The living body undergoes the characteristic changes specified by the soul, and the menstrual fluid is what is properly suited to become an animal. I argue that the strange features that Aristotle ascribes to prime matter in Generation and Corruption are a result of the unusual sort of substantial change that Aristotle is considering. In general, Aristotle thinks that substances have no opposites. However, the elements are characterized by possessing two pairs of opposites: hot/cold, and wet/dry. I argue that this makes changes between the elements in some ways like an alteration, and so the sort of matter needed for changing between the elements will have features that are found in the matter for alteration.