This book develops a novel theory of intentionality. It argues that intentionality is an internal essential relation of constitution between an intentional state and an object, or between such a state and a possible state of affairs as subsisting. The author’s main claim is that intentionality is a fundamentally modal property, hence a non (scientifically) natural property in that it does not supervene, either locally or globally, on its nonmodal physical basis. This is the property, primarily for an intentional mental state, to be constituted by the entities it is about. In the case of intentionality of reference, such constituents are objects, in the sense of individuals; in the case of intentionality of content, such constituents are possible states of affairs as subsisting. Constitution is meant in a mereologically literal sense: those constituents are essential parts of the relevant states. As a result, the theory claims not only that intentionality is relational but also that it is an internal, essential relation holding between an intentional state and its object or proposition-like content. Intentionality as Constitution will be of interest to scholars and advanced students working in philosophy of mind, philosophy of language, metaphysics, and cognitive science.