In this report, we provide an evidence-based overview of attention-deficit/hyperactivity disorder (ADHD), including diagnosis, prevalence, developmental expression of symptoms, persistence, the heterogeneity of functional outcome, impairment in afflicted adults, psychiatric comorbidity, pathophysiology, genetics, psychosocial and biologic risk factors, and neurobiology. Attention-deficit/hyperactivity disorder is an early-onset, highly prevalent neurobehavioral disorder, with genetic, environmental, and biologic etiologies, that persists into adolescence and adulthood in a sizable majority of afflicted children of both sexes. It is characterized by behavioral symptoms of inattention, hyperactivity, and impulsivity across the life cycle and is associated with considerable morbidity and disability. Comorbidity is a distinct clinical feature of both childhood and adult ADHD. Although its etiology remains unclear, emerging evidence documents its strong neurobiologic and genetic underpinnings. Despite the high diagnostic reliability and the robust evidence of the validity of ADHD, there are many underlying issues that remain to be resolved. These include establishing developmentally appropriate diagnostic criteria at older ages, further elaborating the impact of gender on symptom expression, and examining risk and protective factors in relationship to prevention or amelioration of ADHD as well as related functional impairments.