Attention-deficit/hyperactivity disorder (ADHD) is a lifelong neurodevelopmental disorder in which approximately two thirds of patients experience impairment in adulthood. Although some adults with ADHD were diagnosed as children, many are first diagnosed as adults. This poses particular challenges given the limited familiarity with ADHD of many adult mental health services. As a result, several organizations, including the Canadian ADHD Resource Alliance, the American Academy of Child and Adolescent Psychiatry, the National Institutes of Health, and the British Association for Psychopharmacology, have developed practice guidelines for the assessment and treatment of adults with ADHD. This article reviews those guidelines in order to examine current best practices in adult ADHD. There is considerable agreement among these guidelines, which should be a critical part of moving from emerging knowledge to patient care, although both empirical evaluation and ongoing updates as new knowledge emerges will be important for their future development.