Attention-deficit/hyperactivity disorder (ADHD) is a multifactorial and clinically heterogeneous disorder that is associated with tremendous financial burden, stress to families, and adverse academic and vocational outcomes. Attention-deficit/hyperactivity disorder is highly prevalent in children worldwide, and the prevalence of this disorder in adults is increasingly recognized. Studies of adults with a diagnosis of childhood-onset ADHD indicate that clinical correlates--demographic, psychosocial, psychiatric, and cognitive features--mirror findings among children with ADHD. Predictors of persistence of ADHD include family history of the disorder, psychiatric comorbidity, and psychosocial adversity. Family studies of ADHD have consistently supported its strong familial nature. Psychiatric disorders comorbid with childhood ADHD include oppositional defiant and conduct disorders, whereas mood and anxiety disorders are comorbid with ADHD in both children and adults. Pregnancy and delivery complications, maternal smoking during pregnancy, and adverse family environment variables are considered important risk factors for ADHD. The etiology of ADHD has not been clearly identified, although evidence supports neurobiologic and genetic origins. Structural and functional imaging studies suggest that dysfunction in the fronto-subcortical pathways, as well as imbalances in the dopaminergic and noradrenergic systems, contribute to the pathophysiology of ADHD. Medication with dopaminergic and noradrenergic activity seems to reduce ADHD symptoms by blocking dopamine and norepinephrine reuptake. Such alterations in dopaminergic and noradrenergic function are apparently necessary for the clinical efficacy of pharmacologic treatments of ADHD.