When thinking of attention deficit hyperactivity disorder (ADHD), many imagine a mischievous little boy who is constantly restless and rarely listens to what people say to him. That stereotype of ADHD as simply a behavior problem of young children has been replaced with science-based understanding of ADHD as a complex problem in the unfolding development of the brain’s self-management system—its executive functions. ADHD affects about 9% to 11% of children and at least 4% or 5% of adults in the United States. One of the best ways to learn about executive functions impaired in ADHD is to listen to examples provided by persons with ADHD when they describe their difficulties in exercising these functions. This chapter describes six clusters such as, Activation cluster, Focus cluster, Effort cluster, Emotion cluster, Memory cluster, and Action cluster. Assessment for ADHD should also include a review of the Diagnostic and Statistical Manual of Mental Disorders-5 diagnostic criteria for ADHD.