In this thesis I focus on the account that Aristotle gives of sense-perception as an unusual sort of alteration ('DA' II.5, 416b33). If sense-perception is to be considered an alteration then it must involve a change in the qualities of a substance, as the account of alteration in Aristotle indicates. The goal of my dissertation was to see whether we can extract from Aristotle's texts the necessary justifications for calling sense-perception an alteration 'of a sort.' I thus surveyed the terms 'alteration' and 'quality' in the Aristotelian corpus to find out whether it was possible to. The conclusion I obtained is that 'alteration' derives its unusual sense from the primary sense of 'quality,' which is exactly the kind of quality that inheres, as sense-perception does, in the very essence of a being. This finding shows that the account of perception in ' De Anima' II.5 is a succinct statement that is nevertheless consistent and plausible in Aristotle.