OBJECTIVE: The purpose of the study was to examine comorbidity in the context of child and adolescent depression. METHOD: The authors reviewed recent epidemiological studies using standardized interviews and DSM-III or DSM-III-R criteria. RESULTS: There was a high rate of comorbidity in children and adolescents with major depressive disorders or dysthymia. Comorbidity with conduct disorder/oppositional defiant disorder ranged from 21% to 83%; comorbidity with anxiety disorder ranged from 30% to 75%; and comorbidity with attention deficit disorder ranged from 0% to 57.1%. Rates of depressive comorbidity found in community studies were similar to the rates found in clinical studies. In almost all cases, the disorders were more common in depressed children than expected by chance, and the rates of other disorders in depressed children were higher than the rates of depression in those with depression. CONCLUSIONS: The mechanisms by which comorbidity occurs are obscure at present. Several possibilities and their implications for nosology, epidemiology, and treatment research are discussed.