The purpose of this study is to evaluate how children and adolescents with Attention Deficit/ Hyperactivity Disorder (ADHD) respond to treatment with three differing doses of stimulant medications used to treat ADHD, Adderall XR® and Focalin XR®. Another purpose of the study is to evaluate if there are differences in sleep and other side effects, such as changes in mood or loss of appetite, which can occur with stimulant medications. A third purpose is to determine if there are differences in the characteristics of individuals who respond better to either of the medications.