Regular physical activity is consistent with better health including body composition status. More commonly, studies of physical activity and health have focused on the adult years when poor health as a consequence of inappropriate lifestyle behaviors, becomes more evident. In recent years, greater attention has been paid to developmental aspects of health and disease. This includes the notion that early life experiences or exposures to nutrition and movement and subsequent physical activity and exercise behaviors are related to phenotypic changes including predisposition to overweight and obesity at younger ages. The current obesogenic environment is characterized by the coexistence of overnutrition and low levels of physical activity, a situation that has been referred to as an evolutionary mismatch. Overweight and obesity are commonplace at all stages of the lifespan, including during the reproductive years with the prevalence of maternal obesity and gestational diabetes mellitus increasing rapidly along with obesity and diabetes in their offspring. There is an urgent need for effective strategies to break this cycle, ideally by optimizing physical activity and healthy eating in adolescents and young mothers to increase the opportunity of healthy growth and development, including healthy body composition, in infants and young children.