Attention-deficit/hyperactivity disorder or hyperkinetic disorder is a clinically defined syndrome characterised by age inappropriate deficits in sustained attention, impulsivity and overactivity. Despite extensive investigation, a specific neuroanatomical, physiological, biochemical, or psychological origin has not been identified. Diagnosis is based on detailed medical and developmental history, symptom rating scales, psychological assessment and medical evaluation. Increases in diagnosis and treatment of the disorder have elicited public and professional concern. The main focus in this article is on this disorder in children and adolescents and includes practical information on assessment and treatment. Other disorders, which may be either comorbid with or mistaken for hyperkinetic disorder, are reviewed in less detail.