Attention-deficit/hyperactivity disorder (ADHD) is the most common mental disorder in childhood, and primary care clinicians provide a major component of the care for children with ADHD. However, because of limited available evidence, the American Academy of Pediatrics guidelines did not include adolescents and young adults. Contrary to previous beliefs, it has become clear that, in most cases, ADHD does not resolve once children enter puberty. This article reviews the current evidence about the diagnosis and treatment of adolescents and young adults with ADHD and describes how the information informs practice. It describes some of the unique characteristics observed among adolescents, as well as how the core symptoms change with maturity. The diagnostic process is discussed, as well as approaches to the care of adolescents to improve adherences. Both psychosocial and pharmacologic interventions are reviewed, and there is a discussion of these patients' transition into young adulthood. The article also indicates that research is needed to identify the unique adolescent characteristics of ADHD and effective psychosocial and pharmacologic treatments.