For years the assessment and treatment of ADHD seemed relatively simple: a parent or schoolteacher reported that a child, usually a boy, was disruptive, hyperactive, and impulsive, and the pediatrician made the diagnosis. The child was placed on a regimen of stimulant medications, and parents were informed that the disorder would remit at puberty. However, in the past 10 years clinical experience and research studies have supported a new understanding of this disorder. ADHD is a complex illness that affects not only young boys but also young girls, teenagers, and adults. Its cardinal symptom is not necessarily hyperactivity but, rather, symptoms of inattention resulting in disturbances and impairments across a wide range of cognitive functions, including 1) organizing and activating work, 2) sustaining attention and concentration, 3) sustaining energy and effort for work, 4) managing affective interference, and 5) using and accessing information. Each of these skills can be divided and subdivided into skills critical to the attainment of professional and social success.