The book is a systematic study of the issue of self-individuation in the scholastic debate on principles of individuation (principia individuationis). The point of departure is a general formulation of the problem of individuation acceptable for all the participants of the scholastic debate: a principle of individuation of x is what makes x individual (in various possible senses of 'making something individual'). The book argues against a prima facie plausible view that everything that is individual is individual by itself and not by anything distinct from it (Strong Self-Individuation Thesis). The keynote topic of the book is a detailed analysis of the two competing ways of rejecting the Strong Self-Individuation Thesis: the Scotistic and the Thomistic one. The book defends the latter one, discussing a number of issues concerning substantial and accidental forms, essences, properties, instantiation, the Thomistic notion of materia signata, Frege's Begriff-Gegenstand distinction, and Geach's form-function analogy developed in his writings on Aquinas. In the context of both the scholastic and contemporary metaphysics, the book offers a framework for dealing with issues of individuality and defends a Thomistic theory of individuation.