Adolescence is a critical developmental period with long-term implications for the health and well-being of the individual and for society as a whole. The most significant factors to adolescents' health are found in their environments, and in the choices and opportunities for health-enhancing or health- compromising behaviors that these contexts present (e.g., exposure to violence, supportive families). Inadequate contexts represent a failure to invest in and protect adolescents, a choice to alienate rather than integrate them into society. This article describes a number of societal trends, including growing poverty and income disparities, government instability, the changing health-care system, the spread of HIV/AIDS, increased migration and urbanization, changing family and cultural contexts, and new information technology. The health implications of these trends for the well-being of adolescents in the 21st century are contemplated.