This chapter provides a systematic review of research conducted prior to the year 2010, as well as more recent research, examining the relationship between religiosity and mental health in Muslims. Included here are studies examining depression, suicide, anxiety, substance use/abuse, psychotic symptoms, cognitive impairment, and well-being. This comprehensive review finds that reading and reciting the Qur’an, frequent engagement in prayer, holding devout Islamic beliefs, careful adherence to Qur’anic teachings, and a strong and close-knit family and community may help to neutralize feelings of stress and distress and enhance well-being and happiness. Islamic teachings set the bar high in terms of ethical values and behavioral expectations, promising dire consequences in the hereafter for those who fail to reach out to that standard. Nevertheless, Muslims who abide by those teachings appear to have better mental health than those who do not. Clinicians should be aware of these findings, particularly when encountering Muslim patients who are less religious (and those who are religious but may be misunderstanding or misinterpreting Islamic teachings).