Attention-deficit/hyperactivity disorder (ADHD) is characterized by traitlike symptoms, with stable features frequently emerging in early childhood and persisting throughout adolescence and adulthood. The hallmarks are pervasive, developmentally inappropriate levels of inattention, hyperactivity and impulsivity leading to impairments in family life, social life, academic performance, and occupational functioning. Emotional lability is a commonly associated feature that is a further source of impairment. Diagnosis rests on a comprehensive assessment of developmental psychiatric history and current status, following DSM-5 guidelines on ADHD classification and diagnostic thresholds for adults. Medical and psychotherapeutic interventions can improve psychological functioning, self-confidence, personal/family relationships, professional/academic functioning, cognitive performance, driving safety, and quality of life, and may reduce criminal behavior. Future efforts should focus on supporting treatment adherence and smoothing transitions from pediatric to adolescent to adult medical services.