A large number of epidemiological and experimental studies have clearly confirmed in the last years that inactivity is a main cause of disease and that a dose/response relationship exists between physical activity/fitness and global mortality. People who maintain reasonable levels of activity, especially in adult and older ages, have a lower probability of suffering diseases or a premature death. In addition, economic costs in terms of disease, work absentism and health systems, must be considered. It has been calculated that medical cost of active people is a thirty percent lower than that by inactive subjects. Physical activity contributes to longevity and to an improvement of the quality of life by means of physiological, psychological and social effects. In this article therapeutic and preventive effects of physical activity in different diseases and conditions will be revised.