Categories play a major role in contemporary metaphysics. They have not only been invoked in a number of philosophical theories but are themselves objects of epistemological and metaphysical scrutiny. In this article, we will discuss the following questions: How do we know when something belongs to a certain category? Is there a fundamental category of the world? Can we give a satisfactory account of the number of categories and the completeness of systems of categories? Are categories the genuine subjects of (...) ontology? (shrink)