Attention deficit hyperactivity disorder (ADHD) is a common and impairing disorder characterized by developmentally atypical levels of inattention, impulsiveness, and hyperactivity. It starts in childhood and persists into adulthood in about half of affected individuals. ADHD affects many aspects of affected persons' lives and adds substantially to their overall health care costs. Although ADHD starts in childhood, patients may present for the first time in adolescence or adulthood. Neurologists often play a significant role in the assessment and treatment of ADHD at all ages. Assessment should not only evaluate the symptoms of ADHD but also weigh the impairment arising from these symptoms, including associated psychiatric disorders and learning problems, and the presence of precipitating or aggravating medical problems. A number of pharmacologic and nonpharmacologic interventions play a role in treatment.