Attention-deficit-hyperactivity disorder (ADHD) is a common neuropsychiatric disorder that impairs social, academic, and occupational functioning in children, adolescents, and adults. In patients with ADHD, neurobiologic research has shown a lack of connectivity in key brain regions, inhibitory control deficits, delayed brain maturation, and noradrenergic and dopaminergic dysfunction in multiple brain regions. The prevalence of this disorder in the United States is 6-9% in youth (i.e., children and adolescents) and 3-5% in adults. Prevalence rates for youth are similar worldwide. Children with ADHD are at greater risk than children without ADHD for substance abuse and delinquency whether or not they receive drug therapy; however, early treatment with psychoeducation as well as drug therapy and/or behavioral intervention may decrease negative outcomes of ADHD, including the rate of conduct disorder and adult antisocial personality disorder. Drug therapy is effective for all age groups, even preschoolers, and for late-onset ADHD in adults. Stimulants, such as methylphenidate and amphetamine, are the most effective therapy and have a good safety profile; although recent concerns of sudden unexplained death, psychiatric adverse effects, and growth effects have prompted the introduction of other therapies. Atomoxetine, a nonstimulant, has no abuse potential, causes less insomnia than stimulants, and poses minimal risk of growth effects. Other drug options include clonidine and guanfacine, but both can cause bradycardia and sedation. Polyunsaturated fatty acids (fish oil), acetyl-L-carnitine, and iron supplements (for youth with low ferritin levels) show promise in improving ADHD symptoms. As long-term studies show that at least 50% of youth are nonadherent with their drug therapy as prescribed over a 1-year period, long-acting formulations (administered once/day) may improve adherence. Comorbid conditions are common in patients with ADHD, but this patient population can be treated effectively with individualized treatment regimens of stimulants, atomoxetine, or bupropion, along with close monitoring.