Physical fitness is a term describing maximal aerobic capacity adjusted for body size and composition, and is an integrated measure of cardiorespiratory and neuromusculo-skeletal function, oxygen transport and delivery, and psychological drive. Accordingly, high physical fitness requires that all these important body functions function normally, while low physical fitness suggests malfunction of one or more of them. High levels of physical fitness--maintained through heavy daily exercise--has probably been a necessary requirement for survival in the earlier history of humans. In modern, industrialised countries the demand for physical activity to sustain life is declining, and a decline in physical fitness is observed in many populations. Epidemiological studies over the past 50 years have unanimously shown that low physical activity and physical fitness is associated with high cardiovascular and total mortality. Recent data also suggest that low physical activity and physical fitness is followed by an increased incidence of diabetes mellitus and nonfatal cardiovascular diseases. A number of unfavourable biochemical and physiological aberrations following physical inactivity probably explain these observations. However, recent studies also indicate that changes in physical activity, and especially changes that bring increases in physical fitness, can reverse this rather pessimistic scenario. The Global Burden of Disease Study initiated by the World Health Organization included physical inactivity among the most important risk factors threatening global health. A sedentary life style may be as detrimental to health as smoking. Encouragement of physical activity is an important and difficult task, as society is becoming increasingly successful in reducing our need to move.