Research studies over the past several decades confirm the health benefits of regular physical activity, a concept with foundations in antiquity. The effects of activity on certain individual health conditions, the precise dose of activity that is required for specific benefits, the role (if any) of intensity of effort, and the elucidation of biological pathways whereby activity contributes to health are topics for further research. Although details remain to be clarified, it is now clear that regular physical activity reduces the risk of morbidity and mortality from several chronic diseases and increases physical fitness, which leads to improved function. Table 3 outlines the relationship of activity to several diseases, a judgment on the strength of the evidence, and a rough determination of the amount of research extant. Results from clinical exercise studies and epidemiological investigations can be integrated into a consistent and coherent theory of healthful physical activity. However, some differences between these two research streams need to be reconciled. Exercise physiologists have generally recommended relatively intensive activity and a formal approach to exercise prescription. The epidemiological studies suggest a linear dose-response relationship, at least up to a point, between physical activity and health and functional effects. These data support public health recommendations directed toward the most sedentary and unfit stratum of the population and emphasize doing at least moderate physical activity. If this group of adults would accumulate 30 minutes of walking per day (or the equivalent energy expenditure in other activities), they would receive clinically significant health benefits. An important point is that it does not matter what type of physical activity is performed: Sports, planned exercise, household or yard work, or occupational tasks are all beneficial. The key factor is total energy expenditure; if that is constant, improvements in fitness and health will be comparable. There are probably 40 million adults in the US whose sedentary habits place them at considerably increased risk of morbidity and mortality from several diseases. These same individuals also are more likely to have functional limitations, especially as they move into the later years of life. The sizable independent relative risk for impaired health in sedentary persons, and the large number at risk, leads to a substantial public health burden. This problem deserves continued and increased attention by physicians and other health professionals, scientists, and the public health establishment.