Michael J. Loux here presents a fresh reading of two of the most important books of the Metaphysics, Books Z and H, in which Aristotle presents his mature theory of primary substances (ousiai). Focusing on the interplay of Aristotle's early and late views, Loux maintans that the later concept of ousia should be understood in terms of a theory of predication that carries interesting implications for contemporary metaphysics.Loux argues that in his first attempt in identifying ousiai in the Categories, Aristotle encountered a set of ontological problems which he wrestled with again in Metaphysics Z and H. In the Categories, where the primary realities are basic subjects of predication construed in essentialist terms as things falling under natural kinds, familiar particulars are the primary ousiai. In subsequent works, Aristotle holds that since familiar particulars come into being and pass away, they must be composites of matter and form; and in Metaphysics Z and H, he explores the implications of this insight for the search for ousia. Maintaining that the substantial forms of familiar particulars are the primary ousiai, the later Aristotle interprets forms as predicable universals rather than as particulars, each uniquely possessed by a single object.