The goal of the present essay is to understand Aristotle's setup of and solution to the problem of definitional unity, which is given extended treatment in Book VII, Chapter 12 and Book VIII, Chapter 6 of the Metaphysics. I argue not merely that the two chapters can be seen as complementary and mutually consistent, but that each contains a part of the solution not found in the other. In the first two chapters I set the stage for Aristotle's particular problem of unity by contrasting it with related puzzles of definition and of unity found in Plato, and I explain and endorse Aristotle's crucial objections to Forms qua definitional objects. In the next two chapters I explore some of the central machinery of Aristotle's metaphysical system, an understanding of which is required to have a clear conception of the problem of unity. Much of the controversy of the dissertation, and likely with it much of the interest, is to be found in these chapters. I argue here, against the orthodoxy, that, because of Aristotle's insistence (i) that primary things are the same as their essences, (ii) that matter does not belong in a canonical essence and (iii) that essences of primary things consist ultimately of genus and ultimate differentia , one must understand Aristotle's canonical definienda to be not species per se but species-forms, which I further distinguish from differentiae. The resulting view I call “Form-as-Definiendum” (or “FD”), which is to be contrasted with the more traditional “Species-as- Definiendum” (or “SD”) view. The final chapter defends FD against some obvious and powerful objections.