The disorder currently known as attention-deficit disorder (ADD) or attention-deficit/hyperactivity disorder (ADHD) is now recognized by most clinicians as a legitimate and widely prevalent disorder among children and adults. Yet there is still widespread misunderstanding as to the disorder's nature. Many clinicians mistakenly continue to think of this as a behavior disorder characterized by hyperactivity in children and excessive restlessness or impulsivity in adults. In fact, ADD/ADHD is essentially a cognitive disorder, a developmental impairment of executive functions (EFs), the self-management system of the brain. Although EFs are complex, their impairment constitutes a syndrome that can be recognized readily in clinical practice; impaired EF involves a pattern of chronic difficulties in executing a wide variety of daily tasks. Once recognized, this disorder can be effectively treated in most cases. In this article, I describe the nature of EF impairments in ADD/ADHD and how the syndrome can be recognized and effectively treated in clinical practice. (Note: The term ADHD is used in the balance of this article to refer to both inattentive and combined subtypes.).