Adolescent and young adult medicine is a concept that has gained traction in the last decade or so. The medical literature has come primarily from oncology. Advances in neuroscience that document continuing brain development into the third decade, and research that shows risk behaviours associated with adolescence both remain and may increase in the third decade, have been two of the drivers in the conversation around linking these two age groups together as a medical practice group. A third driver of importance is transition care in chronic illness, where older adolescents and young adults continue to have difficulties making effective linkages with adult care. The case for specific training in adolescent and young adult medicine, including the developmental concepts behind it, the benefits of the delineation and the particular challenges in the Australian health-care system, are discussed. On balance, there is a strong case for managing the health issues of adolescents and young adults together. This scenario does not fit easily with the age demarcations that are in place in acute care facilities. However, this is less the case in community services and can work in focused private practice. Such a situation suggests that both paediatric and adult physicians might be interested in adolescent and young adult medicine training and practice.