Attention-deficit/hyperactivity disorder (ADHD) is a psychiatric disorder characterized by inattention, impulsivity, and overactivity that begins in childhood. While considerable research has focused on the neurobiological substrates of this disorder, the specific nature of the brain dysfunction in ADHD has remained elusive. However, early data from pharmacological treatment studies, as well as from basic research in animals and humans, initially led several investigators to develop neurobiological models of ADHD. These models of ADHD and more recent evidence from neuropsychological, neuroimaging, neurochemical, and genetic research are briefly reviewed. While not completely consistent, the empirical data suggest that dysfunction in prefrontal-striatal neural circuits, as well as in brain stem catecholamine systems that innervate these circuits, may underlie the executive function deficits in ADHD.