The mood disorders unipolar major depression and bipolar disorder increase the risk of suicidal ideation, attempted suicide, and death by completed suicide. This article reviews the epidemiologic data on the relationship between mood disorders and suicide, with an emphasis on the substantial risk of suicide, while reassessing older data that may no longer apply. Widespread underdiagnosis and undertreatment of major depression and bipolar disorder contribute to an unacceptable risk of suicide, a preventable tragedy.