Ontology the study of the most fundamental categories of being lies at the very heart of metaphysics. The reason why it appears to be so central is because it takes on the following questions: What sorts of entities are there? What features do those entities have? How do they relate to one another? And so on. Section 1 of this Element presents a fast-paced historical overview of some of the notable approaches to these questions. Section 2 tells the story of how one of the oldest, most disputed, but also most developed ontological categories universals got introduced. Section 3 builds on the discussion of universals as it considers the desiderata for a promising system of ontological categories. And Section 4 looks at ways in which philosophers might break with tradition and explore some new ontological categories.