Physical inactivity is associated with the increase in childhood obesity. Experts recommend that children engage in moderate to vigorous physical activity for a minimum of 60 minutes per day, but how many youth meet this standard is not known. To examine patterns and determinants of moderate to vigorous physical activity among youth, Nader and colleagues analyzed data from a large cohort study of children who had accelerometer-determined physical activity measurements at ages 9 through 15 years. The authors report that at age 9 years, children engaged in moderate to vigorous physical activity for approximately 3 hours per day on both weekdays and weekends. Physical activity steadily declined each year. By age 15 years, adolescents engaged in moderate to vigorous physical activity for 49 minutes per weekday and 35 minutes per weekend. Although boys were consistently more active than girls, rates of decline in physical activity were similar for both boys and girls.