Emerging over the past decade has been the field of comorbidity of substance abuse with other psychological disorders among adolescents. This chapter, examines the comorbid psychiatric disorders interacting with substance use among adolescents and the need for a more sophisticated approach to providing treatment. Signs of the comorbid condition often appear early in life, before the first chemical use. The assessment of comorbidity of adolescent substance abuse with another form of psychopathology remains a two, and sometimes more, step process. Diagnoses are made based on the initial symptomatology presented. Treatment of substance-abusing adolescents with comorbid psychiatric disorders needs to be a team effort led by the direct treatment providers and should include family, school personnel, community agencies, and peers. Assessment and treatment issues related specifically to conduct disorders, Attention Deficit Hyperactivity Disorder (ADHD), mood disorders, and anxiety disorders are examined. Due to the complexity of the problem a multimodal approach has been shown to work the best, providing more effective and humane care. Appendix and references