The prevalence of attention-deficit/hyperactivity disorder (ADHD) has been increasing, and the growing number of people with this disorder, especially the non-pediatric population, get prescriptions for this condition. However, controversies are also growing around this disease. In order to provide optimal treatment to a patient presumed to have adult ADHD, it is crucial to understand the implications of diagnosing and treating adult ADHD. We examined the history of ADHD and changes in the diagnostic criteria postulated by the DSM system (Part I), proposed a critical review on the concept of ADHD from various points of view and suggested the diagnostic and therapeutic implications of adult ADHD (Part II). This study may serve as a small cornerstone for a valid diagnosis and a proper treatment of ADHD, especially in the adult population.