Subjects participating in this study will consist of a total of 220 children and adolescents from 7 to 15 years of age with a gender and ethnic distribution representative of the patient population served by Children's Mercy Hospital (CMH; from a representative 12 month period) as follows: White, 59.6%; African-American, 31.9%; Hispanic, 4.9%; Asian, 0.6%;Native American, 0.1%; Other, 2.9%. The gender distribution is 44.4% female and 55.6% male. An equal number of pediatric patients who meet the DSM-IV criteria for a primary diagnosis of ADHD or ADD (~33%) and age- and sex-matched controls (~67%) will be recruited. The patients represent a population of children who can reasonably expect to be treated with a medication that is primarily dependent upon CYP2D6 for its elimination from the body. An example is atomoxetine, also known as Strattera®. Each patient will be asked to invite a friend to participate in the phenotyping study with them, providing a non-ADHD population as a reference. Additional phenotyping visits will occur every six months (totaling 7 visits). Justification: Assuming a 10% attrition rate for the second visit, the sample size is expected to provide approximately 25 patients and 25 controls (50 subjects total) in each initial 2-year age bin: 7.0 to 9.0 y, 9.1 to 11.0 y, 11.1 to 13.0 y, and 13.1 to 15.0 y.