In the past decade, a remarkable series of experiments from laboratories all over the world has begun to show what causes aging—and how to slow it. In the latest example of such aging research, two of my Harvard Medical School colleagues, cardiologist Richard T. Lee (co-editor in chief of the Harvard Heart Letter) and stem cell biologist Amy Wagers and their teams have found a substance that rejuvenates aging hearts in mice. The researchers joined the circulation of an old mouse with a thick, stiffened heart to that of a young mouse. After four weeks, the heart muscle of the old mouse became dramatically thinner and more flexible. The team then identified a substance called growth differentiation factor 11 (GDF11) as the probable “anti-aging” substance. It’s too soon to tell if this discovery will ever help humans with heart failure. But it reveals that there are substances naturally present in all living things that cause aging and that retard it. By understanding them, we may someday be able to slow aging.