The medical benefits of regular physical activity, including weight loss and reduction in the risk of heart disease and certain cancers, are well known. Physicians are still reluctant, however, to prescribe exercise for their patients. Although many cite lack of time or poor reimbursement for counseling services, I believe the majority of primary care physicians are simply unsure of how to effectively begin discussing exercise with their patients. This article will review the medical benefits of exercise, basic principles of physiology, and then the components of an exercise prescription. With 250,000 deaths each year attributed to a sedentary lifestyle, it is incumbent upon physicians to include a discussion of regular physical activity with their patients, at every visit.