Recent advances in understanding the developmental processes associated with the onset or persistence of childhood and adolescent problems warrant new thinking about social policies and programs. We now know more than we ever have about why some children and adolescents develop social and health problems, and in the case of problems such as sexually transmitted diseases, drug use, and delinquency, why they appear to make choices that lead to poor outcomes at home and in the school and community.Unfortunately, this knowledge is not yet systematically applied to policy or program design; the result is poorly integrated and often duplicative services for children and families. The motivation for this book comes from the growing recognition that knowledge gained from understanding the developmental trajectories of children who experience social and health problems must be used to craft more effective policies and programs for children and their families. In this book, we argue that a public health framework rooted in ecological theory and based on principles of risk, protection, and resilience is a useful conceptual model for the design of social policy across the substantive areas of child welfare, education, mental health, health, developmental disabilities, substance use, and juvenile justice. Recommendations for ways to advance a public health framework in policy design, implementation, and evaluation are offered.