Atomoxetine is an FDA-approved medication that is used for children with Attention Deficit Hyperactivity Disorder (ADHD). Researchers at Children’s Mercy Hospital & Clinics want to find ways to better use atomoxetine (Strattera®). Some children behave better in school and pay attention in class while taking this medicine. However, for other children, atomoxetine does not seem to help with their ADHD symptoms. This study focuses on understanding how genetic differences impact how well atomoxetine works in an individual child. The goal of this study is to learn how to ensure that a dose of atomoxetine is not too big, not too small, but just right for every child with attention deficit/hyperactivity disorder (ADHD).