The diagnosis of attention-deficit/hyperactivity disorder (ADHD) in adults has remained controversial. This paper reviews the empirical evidence to date as to whether the diagnosis of ADHD in adults is valid and consistent with the childhood syndrome. Evidence of descriptive, divergent, predictive, and concurrent validity were examined. The available literature provides evidence that adult ADHD can be reliably diagnosed and that the diagnosis confers considerable power to forecast complications and treatment response. Studies of genetic transmission, specific treatment responses, and abnormalities in brain structure and function in affected individuals are also consistent with studies in childhood ADHD. There is converging evidence that adult ADHD is a not rare, valid clinical diagnosis. In addition, studies suggest that adult and child patients with ADHD may share a similar treatment-responsive, underlying neurobiological substrate.