The extant adoption, twin, and family studies of suicide and suicidal behavior are reviewed. Suicidal behavior is highly familial, and on the basis of twin and adoption studies, heritable as well. Both completed and attempted suicide form part of the clinical phenotype that is familially transmitted, as rates of suicide attempt are elevated in the family members of suicide completers, and completion rates are elevated in the family members of attempters. A family history of suicidal behavior is associated with suicidal behavior in the proband, even after adjusting for presence of psychiatric disorders in the proband and family, indicating transmission of attempt that is distinct from family transmission of psychiatric disorder. Impulsive aggression in probands and family members is associated with family loading for suicidal behavior, and may contribute to familial transmission of suicidal behavior. Shared environment effects such as abuse, imitation, or transmission of psychopathology are other possible explanations.