Is aging necessary? Are the wrinkles and gray hair, weakening muscles, neurodegeneration, reduced cardiovascular function, and increased risk of cancer that afflict organisms toward the end of their lives inevitable? Or are these age-related changes part of a genetic program that can be altered? Molecular biologists experimenting with organisms such as yeast, roundworms, fruit flies, and mice have found that they can dramatically extend life span by tweaking single genes. The altered organisms don’t just live longer, they age more slowly, in many cases retaining youthful characteristics even after normal individuals have died. More remarkable, the genetic manipulations that cause these changes seem to work through a common pathway across all species. This suggests that if there is a program that controls aging, it must be ancient indeed: in evolutionary terms, yeast and mammals diverged about a billion years ago.