Physical activity across the lifespan makes a clear, scientific case for exercise, sports, and an active lifestyle in preventing illness and establishing lifetime health habits at both the individual and the population levels. The book focuses on key aspects of physical/mental well-being—weight, mood, and self-regulation—and the role of physical activity in public health and school-based interventions targeting these areas. Contributors review definitional and measurement issues salient to understanding what physical activity is, to analyzing benefits of participation, and to implementing effective interventions. Also addressed are limitations of current research, steps needed to continue building the field, and emerging therapeutic possibilities for activity, such as the role of rough and tumble play in preventing ADHD.