Sedentary lifestyle is associated with obesity and obesity-related chronic diseases. Increased physical activity can produce clinically significant changes in body composition and a reduction in risk of obesity-related chronic diseases in obese people. Because a great proportion of a primary care physician's patient population will be obese, it is essential that physicians know the scientific facts about the role of physical activity in the treatment of obesity and how to effectively prescribe it. This article summarizes significant research conclusions about the relationship between physical activity, obesity, and health. It also provides practical strategies for helping obese patients integrate physical activity into their lives.