The case for investing in the health and wellbeing of the largest generation of adolescents in history could be considered self-evident. Failure to do so would counteract hard-won gains from investing in maternal and child health and would risk deteriorating prospects for young people at the threshold of adult life. Yet stark inequalities between and within countries in the education, employment, and health and wellbeing of adolescents undermine their hopes and aspirations and threaten the economic foundations and political stability of societies.