Young people in their teens constitute the largest age group in the world, in a special stage recognized across the globe as the link in the life cycle between childhood and adulthood. Longitudinal studies in both developed and developing countries and better measurements of adolescent behavior are producing new insights. The physical and psychosocial changes that occur during puberty make manifest generational and early-childhood risks to development, in the form of individual differences in aspects such as growth, educational attainment, self-esteem, peer influences, and closeness to family. They also anticipate threats to adult health and well-being. Multidisciplinary approaches, especially links between the biological and the social sciences, as well as studies of socioeconomic and cultural diversity and determinants of positive outcomes, are needed to advance knowledge about this stage of development.