Physical activity clearly is associated with decreased risk of many chronic diseases, as well as with longer life. Utilizing modern epidemiologic methods, studies of physical activity and health have been conducted since the 1940s. However physical inactivity did not gain widespread acknowledgement as a major risk factor for poor health until 1992, when the American Heart Association recognized it as a risk factor for heart disease, on par with risk factors such as smoking. This text includes chapters describing the associations between physical activity and major diseases. With a major emphasis on the methods underpinning studies that can be conducted to elucidate these associations, this book is an important guide for those performing the informative epidemiologic studies needed to reduce the increasing number of people diagnosed with chronic disease due to inactivity.