Attention-Deficit /Hyperactivity Disorder(ADHD), previously considered as a disease of childhood, is a disorder characterized by inattention, hyperactivity, and impulsivity. Recent studies, however, revealed that in about a half of the ADHD children the patients continue their symptoms into their adulthood, which implicates that ADHD is not a childhood disorder but a disorder that a patient has to live with for his or her life. The prevalence of ADHD is 2~6% of general population, with a male preponderance. Since ADHD usually develops in school ages, patients experience such problems as learning difficulties, maladjustment, depressive episodes, substance use, and conflicts with parents or other children. Co-morbidities, e.g. tic, anxiety conduct, and oppositional defiant disorders also exist. Although the exact etiology of ADHD remains to be elucidated, abnormalities of specific areas and pathways in the central nervous system, which is reflected by the term, ADHD-minimal brain damage or minimal brain dysfunction in the early 20th century, have been reported. Genetic factors also have to be considered. To establish the diagnosis of ADHD, the behavior of children and collateral information are required. Objective tests including intelligence, neuropsychological tests, and continuous performance test are helpful. Pharmacotherapy and family education are the most important elements in treating ADHD patients. Cognitive-behavior therapy, play therapy, and social skill training are also useful. A CNS stimulant such as methylphenidate is the most commonly prescribed drug, and the prescription of newly developed drugs, atomoxetine and modafinil, is increasing. The success of treatment depends on both medication and management of psychological and behavioral problems. Early diagnosis and intervention can improve the prognosis of the patients.