Recent decades have seen the emergence of various cognitive and biological explanations of religious belief that claim to be better scientifically supported than predecessor explanations. This article provides an overview of such explanations and some of the philosophical discussions they have evoked. Contemporary naturalistic explanations of religious belief come in three types: (1) cognitive explanations, (2) evolutionary explanations and (3) co-evolutionary explanations. Some writers have claimed that scientifically plausible biological and psychological accounts of religious belief make religious belief itself irrational because they reveal the naturalness and unreliability of the mechanisms that produce religious belief. Conversely, others have argued that the possible naturalness of religious belief is not enough to make it irrational or unwarranted. They maintain that naturalistic explanations of religious belief cannot debunk religious belief wholesale, but instead their religious relevance depends on the ways in which religious people justify their beliefs. Philosophical debates are currently in their early stages, as are the naturalistic theories, and the chief philosophical stakes themselves have not yet come entirely into focus.