Attention-deficit/hyperactivity disorder (ADHD) is commonly perceived as a childhood disorder, but ADHD symptoms can persist into adulthood. The symptoms, deficits, and consequences associated with adult ADHD can have a profound impact on the daily lives of patients and their families. Self-awareness of ADHD symptoms can sometimes be difficult for adults who have lived with these symptoms since childhood. Attention-deficit/hyperactivity disorder is highly comorbid with other psychiatric disorders, and physicians should maintain a high index of suspicion and integrate screening for ADHD when they feel that psychiatric evaluation is indicated in their adult patients. Attention-deficit/hyperactivity disorder can be screened for in as little as 3 to 5 minutes with a variety of self-report or physician-implemented scales. An accurate diagnosis of ADHD in adults requires a comprehensive clinical interview that reviews representative symptoms starting in childhood that continue to cause impairment into adulthood. It is often helpful to interview or obtain information from family, friends, coworkers, or school records. With increasing understanding of ADHD in adults, the recent development of adult-specific diagnostic scales promises further improvement in the standard of care for this disorder. Appropriate treatment of ADHD in adults can provide dramatic symptomatic improvement with some of the largest treatment effects of any medical condition.