Attention Deficit Hyperactivity Disorder (ADHD) is common among adolescents seeking treatment for marijuana dependence. Therefore, it is important to develop treatments targeting adolescents diagnosed with both ADHD and marijuana dependence. Atomoxetine is currently used to treat individuals diagnosed with ADHD. The purpose of this study is to determine the feasibility of administering atomoxetine in a therapeutic setting to adolescents who abuse marijuana and have ADHD. In addition, this study will assess whether atomoxetine reduces ADHD symptoms. Finally, this study will determine whether atomoxetine improves treatment retention and progress in adolescents with ADHD and marijuana dependence.