Attention Deficit Hyperactivity Disorder (ADHD) is common among adolescents seeking treatment for marijuana dependence. The purpose of this study is to determine the use of atomoxetine in treating adolescents who abuse marijuana and have ADHD. In addition, this study will assess whether atomoxetine reduces ADHD symptoms as compared to a placebo. Finally, the study will determine whether atomoxetine improves treatment retention and progress in adolescents with ADHD and marijuana dependence.