In this second in a series of articles on religion and medicine, I focus on the relationship between religion and mental health. This discussion is based on a comprehensive and systematic review of a century of research examining religion's relationship to mental health, social support, substance abuse, and other behaviors affecting mental or social functioning. This review includes over 630 separate data-based reports that focus on religion and well-being, hope and optimism, meaning and purpose, depression, suicide, anxiety, psychosis, social support and marital stability, alcohol and drug abuse, cigarette smoking, extra-marital sexual behaviors, and delinquency. Reasons for the associations found are discussed and conclusions drawn in light of the findings.