We've all heard the mantra, exercise for good health. In fact, exercise, or lack of it, may be the most important factor in avoiding, or surviving and recovering from, the top three killers across developed countries-heart disease, cancer, and stroke. But few of us understand exactly how different forms of exercise work-physiologically speaking-to keep us healthy and prevent or treat disease. Here, two nationally known exercise experts lead a stellar team explaining, in reader-friendly terms, what exercise does to our bodies and how it spurs beneficial biological actions.