I've never understood or agreed with people who don't run because it "hurts the knees." Or worse, they don't run anymore because being a runner in high school and/or college "ruined their knees." If running hurts your knees, it's likely you're just doing it wrong. The "heel strike" is the primary cause of pain. Runners, true runners, run on the balls of their feet, and it's the quads and calf muscles that absorb the shock. Thus, the knee is not the target of force in running. Knees should have little to do with it. And, these days there is an ever-growing body of research that supports the idea that running is actually good for your knees. Gretchen Reynolds of the New York Times spotlights the research that speculates running not only won't ruin your knees but is actually good for them, including the idea that it may prompt cartilage self repair.