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.