Suicide is the third leading cause of death in adolescence, and medically serious suicide attempts occur in approximately 3% of adolescents. This review examines a number of risk factors that contribute to suicidal behavior. A prior suicide attempt is one of the best predictors of both a repeat attempt and eventual completed suicide. Depression, disruptive behavior disorders, and substance-use disorders also place adolescents at high risk for suicidal behavior, with comorbidity further increasing risk. Research on families indicates that suicidal behavior is transmitted through families. Groups at high risk for suicidal behavior include gay, lesbian, and bisexual youths, incarcerated adolescents, and homeless/runaway teens. Although abnormalities in the serotonergic system have not been consistently linked to suicidal behavior, genetic and neurobiologic studies suggest that impulsive aggression may be the mechanism through which decreased serotonergic activity is related to suicidal behavior. Findings from prevention and intervention studies are modest and indicate the need for substantially more theory-driven treatment research.