Adolescent substance abuse results in significant negative outcomes and extraordinary costs for youths, their families, communities, and society. Moreover, rates of psychiatric comorbidity among substance abusing youth are high and youths with a dual diagnosis are more costly to treat. At the time of this study, however, no outpatient treatments had been tested specifically for treating youth with co-occurring substance use disorders and mental health diagnoses. The current project aimed to adapt and evaluate Multisystemic Therapy (MST), a well validated treatment for chronic behavioral problems or serious emotional disturbance in adolescents, to treat dually diagnosed youth using an outpatient model of service delivery.