Worldwide, more than 10 million individuals are in prison at any given time, and more than 30 million individuals circulate through prison each year. Research has consistently shown that prisoners have high rates of psychiatric disorders, and in some countries more people with severe mental illness are in prisons than in psychiatric hospitals. Despite the high level of need, these disorders are frequently underdiagnosed and poorly treated. In this structured review, we provide an overview of the epidemiology of psychiatric disorders in prison, summarise information on rates of suicide and violence victimisation and risk factors for these outcomes, and outline evidence-based interventions for mental health care. Based on this review, we propose a series of clinical, research, and policy recommendations. The aim is to provide a broad synthesis of the main issues related to the mental health of adult prisoners, and highlight gaps in evidence and practice. Two special populations are briefly discussed, namely women and older adults. Juveniles in prison have distinct mental health needs, and an overview of these is outside the scope of this Review.