Physical activity is emerging as an important modifiable disease risk factor in developing countries. This notwithstanding, the effects of physical activity on health outcomes remain incompletely understood; however there is ample evidence that physical activity is associated with low body weight and low fat mass. Furthermore, assessment of energy expenditure in free-living subjects is central to complete understanding of the aetiology of obesity, diabetes, hypertension, coronary heart disease, and osteoporosis amongst other lifestyle-related disorders. Recent studies have indicated a clear epidemiological transition in disease profiles in Africans with an increasing prevalence in lifestyle related disorders such as obesity and related co-morbidities. There has also been a clear trend in these lifestyle disorders with the development of urbanization and consequently; adoption of Westernized lifestyles associated with decline in physical activity due primarily to mechanization/automation of occupational and leisure time activities. Furthermore, there is also developing interest in the concept of sedentary behaviour. It is now increasingly accepted that sedentary behaviour is not simply a lack of physical activity but is an independent behaviour (TV/computer use, reading, homework, etc.), which constitutes a potential risk to health irrespective of physical activity level. Current empirical evidence linking physical inactivity and health outcomes is substantial.