physical activity is defined as any bodily movement produced by skeletal muscle that results in energy expenditure. The doubly labeled water method for the measurement of total energy expenditure (TEE), in combination with resting energy expenditure, is the reference for physical activity under free-living conditions. To compare the physical activity level (PAL) within and between species, total energy expenditure is divided by resting energy expenditure resulting in a figure without dimension. The physical activity level for sustainable lifestyles ranges between a minimum of 1.1-1.2 and maximum of 2.0-2.5. The average physical activity level increases from 1.4 at age 1 year to 1.7-1.8 at reproduction age and declines again to 1.4 at age 90 year. Exercise training increases physical activity level in young adults when energy balance is maintained by increasing energy intake. professional endurance athletes can reach physical activity level values around 4.0. Most of the variation in physical activity level between subjects can be ascribed to predisposition. Most of the variation in physical activity level between subjects can be ascribed to predisposition. A higher weight implicate higher movement costs and less body movement but not necessarily a lower physical activity level. Changes in physical activity primarily affect body composition and to a lesser extent body weight. Modern man has a similar physical activity level as a wild mammal of a similar body size.