The concept of an ontological category is central to metaphysics. Metaphysicians argue about which category of existence 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, and Jan Westerhoff now provides the first in-depth analysis. After examining a variety of attempted definitions, he proceeds to argue for a new understanding of ontological categories, according to which they are systematizations of our knowledge of the world rather than essential characteristics of the world itself. Metaphysicians will find his work highly stimulating.