Although ancient thinkers suggested that physical activity is good for health, systematic research on the topic did not begin until the middle of the 20th century. Early reports showed that individuals in active occupations had lower rates of heart disease than individuals in sedentary occupations. Investigators then began to evaluate leisure-time physical activity and health and found similar results. Later research used objective measures of cardiorespiratory fitness as the exposure, and found even stronger associations with health outcomes. Recent research has extended the earlier findings on activity or fitness and heart disease to a wide variety of health outcomes. We now know that regular physical activity of 150 minutes/week of moderate intensity physical activity reduces the risk of numerous chronic diseases, preserves health and function (both physical and mental) into old age, and extends longevity. The current challenge is to develop programs and interventions to promote physical activity for all in our increasingly sedentary societies.