This chapter explores the metaphysics of abstract objects, that is entities that transcend sensory interaction but turn out to be fundamental in many domains: for example, prima facie, we deal with abstract entities in mathematics, literature, and language. The chapter discusses the distinction between abstract (e.g., numbers) and concrete objects (e.g., chairs) according to the fourfold approach discussed by Lewis. The most prominent distinction criteria hinge on spatiotemporal location, causal efficacy, and abstraction processes. We discuss, moreover, the problem of a metaphysical taxonomy for entities like games, languages, and fictional characters as abstract. The peculiarities of these entities is that they depend on human activity and, at the same time, exhibit some traits typical of abstract entities, for example, because some of them lack spatial—but not temporal—location. The last part of the chapter is dedicated to the problem whether abstract entities exist. In particular, the tension is discussed between nominalism and realism with reference to the existence of mathematical objects.