In recent years, the validity of attention-deficit/hyperactivity disorder (ADHD) in adulthood has gained acceptance among mental health researchers and clinicians. This article will outline the history of this process, provide our best understanding of the characteristics of ADHD in adulthood, review current guidelines and controversies in the assessment of adult ADHD, and summarize pharmacological and psychosocial treatment options. Despite the strides gained in understanding adult ADHD, the authors encourage more research on this population and caution that current conceptualizations of the disorder are based on a limited amount of empirically based knowledge.