Suicide is a complex outcome of multiple, inter-related factors. This article presents the epidemiology of completed and attempted suicide and discusses the known risk factors for suicide within a framework designed to encourage a systematic approach to theory testing and prevention. Mental and addictive disorders, frequently in co-occurrence, are the most powerful risk factors for suicide in all age groups, accounting for over 90 percent of all completed suicides. In combination with proximal risk factors such as access to firearms or other lethal means, recent and severe stressful life events, and intoxication, they can form the necessary and sufficient conditions for suicide.