Psychopathology coexisting with substance abuse in adolescents is often encountered in a variety of clinical settings. Research findings suggest a major role for substance use in the etiology and prognosis of psychiatric disorders such as affective disorders, conduct disorder and antisocial personality disorder, attention-deficit hyperactivity disorder, and anxiety disorders. Psychiatric disorders also appear to have an important role in the etiology of and vulnerability to substance use problems in adolescents. Although the comorbidity of substance abuse and other psychiatric disorders in adolescents is recognized as an important factor in the treatment of adolescents, further research is needed to establish its prevalence, genetics, and clinical implications.