A realist approach to metaphysics and ontology is defended in the face of some antirealist tendencies in contemporary philosophical thought. The general notion of an ontological category is explained and justified. Different systems of ontological categories are compared and contrasted with the four-category ontology: a one-category ontology of modes or tropes, a two-category ontology of particulars and universals, and a two-category ontology of substantial particulars and modes. The ontological status of states of affairs and natural laws, and the ontological implications of the truthmaker principle as advocated by D. M. Armstrong are discussed.