This review discusses key findings and recommendations related to the role of physical activity in weight gain prevention, weight loss, and weight-loss maintenance across the lifespan. For weight gain prevention, epidemiological and clinical studies suggest that regular physical activity is critical, with increases above the recommended 30 min daily for health (e.g., to 45 to 60 min daily) potentially desirable for curtailing weight gain. For weight loss, clinical studies suggest that physical activity interventions alone usually produce only modest results. Combining physical activity with dietary interventions increases the chance of success, especially at higher levels of physical activity (e.g., 200 to 300 min or more weekly). For weight-loss maintenance, high levels of physical activity (e.g., 40 to 90 min daily) may be necessary. To manage weight across the lifespan, a comprehensive approach to physical activity is needed supported by public policy interventions that help make physical activity a part of daily life.