This article examines the main lines of the doctrine of substance (ousia) advanced in Aristotle’s Categories, ch. 5. I discuss the relation between language and the categories (§ 1); the meaning and applications of the term ousia in Plato and Aristotle (§ 2); the role of “primary”, individual substances as ultimate subjects of predication and their relation to their essential and non-essential attributes (§§ 3–4); the metaphysical status of “secondary”, universal substances (§ 5); the different ways in which a primary and a secondary substance is one (§ 6); the metaphysical primacy of primary substances and the dependence of other entities on them (§ 7); the relation between substances and those among their parts which are themselves substances (§ 8); other characteristics of substance, namely not having a contrary and not admitting of “the more and the less” (§ 9); the relation between substance and change (§ 10).