This article presents the epidemiology of suicide with a special focus on suicides among the elderly, and discusses the known risk factors for suicide within a framework designed to encourage a systematic approach to theory testing and prevention. Throughout the world, suicide rates are highest among the elderly. The risk factors for suicide can be classified as distal or proximal, and, within these broad categories, as sociodemographic, psychiatric, biological, familial, and situational. Mental and addictive disorders are the major risk factors for suicide in all age groups. Other risk factors include male gender, disrupted marital status, prior suicide attempt, reduced brain stem serotonergic activity, family history of psychiatric disorder or suicide, a firearm in the home, and a recent, severely stressful life event. Since risk factors for suicide rarely occur in isolation, prevention efforts are more likely to succeed if multiple risk factors are targeted.