Investment in the capabilities of the world's 1·2 billion adolescents is vital to the UN's Sustainable Development Agenda. The study examined investments in countries of low income, lower-middle income, and upper-middle income covering the majority of these adolescents globally to derive estimates of investment returns given existing knowledge. Investments in health and education will not only transform the lives of adolescents in resource-poor settings, but will also generate high economic and social returns. These returns were robust to substantial variation in assumptions. Although the knowledge base on the impacts of interventions is limited in many areas, and a major research effort is needed to build a more complete investment framework, these analyses suggest that comprehensive investments in adolescent health and wellbeing should be given high priority in national and international policy.