This book provides an exhaustive overview of the ontology of relations. Moreover, it offers a detailed defense of the existence of irreducible relations in the universe and shows that entities such as powers should be better thought of as relations. At first, the author discusses many classical arguments for and against the existence of relations and draws preliminary distinctions between internal and external relations and symmetrical and non-symmetrical relations. He defends the existence of irreducible relations against several objections, most notably three Bradleyan regresses. In response to these objections, the author argues that both internal and external relations should be thought of as relational modes and that intentional, temporal and spatial relations are external. He also presents several problems that are typically taken to affect non-symmetrical relations and introduces one new version of a theory to deal with them—the O-Roles theory. Finally, the author explores essential dependence relations and causal dependence relations and defends the view that powers are relational modes. The Ontology of Relations will be of interest to scholars and advanced students working in metaphysics, philosophy of science, and history of philosophy.