This presentation is drawn from Dr. Barkley\\\'s new book on ADHD in Adults: What the Science Says (October, 2008), in which he describes the results of two of the largest studies of ADHD in adults. One of these studies is the Milwaukee follow-up study that he has been conducting since 1978. New findings from this study are presented along with those of other follow-up studies concerning the impact of ADHD on various major life activities during adolescence and by adulthood. These include risk for psychiatric disorders, educational functioning, occupational functioning, financial management, driving, social relationships, risky sexual activities, and health behavior. He also discusses the treatment implications that arise from these findings.