The concept of an ontological category is central to metaphysics. Metaphysicians argue about which category an object should be assigned to, whether one category can be reduced to another one, or whether there might be different equally adequate systems of categorization. Answers to these questions presuppose a clear understanding of what precisely an ontological category is, an issue which is rarely addressed; Jan Westerhoff presents the first in-depth analysis both of the use made of ontological categories in the metaphysical literature, and of various attempts at defining them. He also develops a new theory of ontological categories which implies that there will be no unique system, and that the ontological category an object belongs to is not an essential property of that object. Systems of ontological categories are structures imposed on the world, rather than reflections of a deep metaphysical reality already present. All metaphysicians should find Westerhoff's book highly stimulating.