Attention-deficit/hyperactivity disorder (ADHD) is the most common childhood psychiatric disorder and probably one of the most common undiagnosed psychiatric disorders in adults. ADHD always begins in childhood and may be followed by a lifelong expression of symptoms, including inattention, excessive motor hyperactivity or restlessness, and poor impulse control. Standard measures exist for evaluating the disorder and assessing treatment effects. Children and adults are highly responsive to pharmacotherapy and may benefit from behavioral and psychosocial interventions.