My title, “Belief in Naturalism,” signals, not that I adopt naturalism as an article of faith, but that my purpose in this paper is to shed some light on what belief is, on why the concept of belief is needed in epistemology, and how all this relates to debates about epistemological naturalism. After clarifying the many varieties of naturalism, philosophical and other (section 1), and then the various forms of epistemological naturalism specifically (section 2), I offer a theory of belief in which three elements – the behavioral, the neurophysiological, and the socio-historical – interlock (section 3), and apply this theory to resolve some contested questions: about whether animals and pre-linguistic infants have beliefs, about the fallibility of introspection, and about self-deception (section 4).