Participants in this double-blind study will be randomly assigned to receive either atomoxetine or placebo. Study visits will occur 2 times each week for the duration of the 12-week trial. At each study visit, vital signs and medication side effects will be assessed. In addition, psychiatric ratings measurements will be evaluated. Participants will also complete self reports in order to track ADHD-related symptoms. Starting at the Week 2 study visit, all participants will receive psychotherapy sessions. This will involve cognitive-behavioral therapy/relapse prevention treatment that has been designed specifically for ADHD adolescents. The aim of the cognitive skills training will be to assist the participants in developing the skills necessary to manage their drug use as well as the symptoms they experience related to ADHD. At Month 6, a follow-up visit will occur, at which time participants urine toxicology tests will be completed to determine if participants have been using marijuana or any other substances of abuse. In addition, at the Month 6 follow-up visit, participants will complete psychiatric ratings, self reports, and will meet with a physician to assess ADHD-related symptoms.