This chapter presents an extensive literature review and data from a sample of more than 30,000 children and youth from 30 countries, showing that young people worldwide do not experience an adequate level of developmental relationships with nonfamily adults that feature (a) care, (b) challenge, (c) support, (d) sharing of power with adults, and (e) expansion of young people’s possibilities. Young people who experience high-quality developmental relationships with nonfamily adults are significantly better off on a variety of well-being indicators, including positive identity, workforce readiness, educational attainment, spiritual development, and sexual responsibility. It is concluded that nonfamily adults represent a vast, largely untapped, resource for positive youth development and well-being globally. Implementing policies and practices to measure, track, and build those developmental relationships may be a relatively low-cost way to both promote youth well-being and efficiently multiply the positive impact of existing international aid and humanitarian investments.