A special issue of The Lancet focuses on the health and wellbeing of the world’s 1.8 billion adolescents and young adults, the largest generation the world has seen and one that faces unprecedented social, economic, and cultural change. The issue explores the current threats facing this age group, but also the unrealized opportunities for the future of society. The publication was produced by the MacArthur-supported Lancet Commission on Adolescent Health and Wellbeing, formed to address why rapid improvements in child health and survival in many countries is not being matched by improvements in the health of adolescents and young adults, ages 10 to 24. The commission will consider next steps needed to implement better health services and consider how health service systems can better this generation of young people.