There is increasing scientific recognition that attention-deficit/hyperactivity disorder (ADHD), a heterogeneous disorder that carries a high risk of comorbidity, continues past childhood and adolescence into adulthood in many cases and may be underidentified in girls. The etiology of ADHD is unknown, although evidence from family studies of ADHD suggests a genetic origin for some forms of this disorder. A variety of pharmacologic agents are available in treating ADHD: stimulant medications remain the first-line treatment for noncomorbid ADHD, whereas tricyclic antidepressants and bupropion are recommended for stimulant nonresponders and patients with more than one psychiatric disorder. Complex cases of ADHD, however, may require rational use of combined pharmacotherapy.