This paper reviews evidence for a relationship between religion and health. Hundreds of epidemiologic studies have reported statistically significant, salutary effects of religious indicators on morbidity and mortality. However, this does not necessarily imply that religion influences health; three questions must first be answered: "Is there an association?", "Is it valid?", and, "Is it causal?" Evidence presented in this paper suggests that the answers to these respective questions are "yes," "probably," and "maybe." In answering these questions, several issues are addressed. First, key reviews and studies are discussed. Second, the problems of chance, bias, and confounding are examined. Third, alternative explanations for observed associations between religion and health are described. Fourth, these issues are carefully explored in the context of Hill's well-known features of a causal relationship. Despite the inconclusiveness of empirical evidence and the controversial and epistemologically complex nature of religion as an epidemiologic construct, this area is worthy of additional investigation. Further research can help to clarify these provocative findings.