Religious and spiritual factors are increasingly being examined in psychiatric research. Religious beliefs and practices have long been linked to hysteria, neurosis, and psychotic delusions. However, recent studies have identified another side of religion that may serve as a psychological and social resource for coping with stress. After defining the terms religion and spirituality, this paper reviews research on the relation between religion and (or) spirituality, and mental health, focusing on depression, suicide, anxiety, psychosis, and substance abuse. The results of an earlier systematic review are discussed, and more recent studies in the United States, Canada, Europe, and other countries are described. While religious beliefs and practices can represent powerful sources of comfort, hope, and meaning, they are often intricately entangled with neurotic and psychotic disorders, sometimes making it difficult to determine whether they are a resource or a liability.