Frequently, substance abuse treatment is unavailable to youths in the juvenile justice system, and when treatment is available, it may be provided in group format using untested therapies. A motivation/skills-based intervention (delivered in group format) may prove efficacious in enhancing motivation and in reducing substance abuse and related problems. This study extends previous research by rigorously evaluating group treatment for incarcerated teens. We will examine processes contributing to the efficacy of group MI/CBT, and the influence of race and ethnicity on treatment effects. The development of effective interventions for substance using juvenile offenders has the potential to reduce substance abuse and crime in this population.