Both science and philosophy are interested in questions of ontology - questions about what exists and what these things are like. Science and philosophy, however, seem like very different ways of investigating the world, so how should one proceed? Some defer to the sciences, conceived as something apart from philosophy, and others to metaphysics, conceived as something apart from science, for certain kinds of answers. This book contends that these sorts of deference are misconceived. A compelling account of ontology must appreciate the ways in which the sciences incorporate metaphysical assumptions and arguments. At the same time, it must pay careful attention to how observation, experience, and the empirical dimensions of science are related to what may be viewed as defensible philosophical theorizing about ontology. The promise of an effectively naturalized metaphysics is to encourage beliefs that are formed in ways that do justice to scientific theorizing, modeling, and