Attention-deficit/hyperactivity disorder (ADHD) is characterized by an age-inappropriate level of inattention, with or without impulsivity and overactivity, that occurs across settings, causes functional impairment, and cannot be attributed primarily to another disorder. It begins in childhood, and although the observable symptoms change in quality and quantity across development, most individuals with ADHD continue to experience some symptoms as adults. Because of its prevalence, persistence, the functional impairment it causes, and the labor-intensive nature of its management, ADHD is extremely important to affected individuals, their families, our health and educational systems, and for society as a whole.

ADHD is a subject that generates strong feelings and much controversy. It is a purely behaviorally defined disorder, without a specific biologic marker, and with characteristics that occur along a continuum across the population. It can occur with or without several commonly associated conditions. It is no wonder that ADHD is a challenge for children, families, educators, clinicians, and researchers alike. The goal of this article is to provide current background for the practicing clinician regarding the clinical, neuropsychological, and neurobiologic correlates of ADHD. This background should provide a base from which to keep abreast of new developments as researchers bring us closer to the answer to the question, “What is ADHD?”