Wild animals seldom survive long enough to age. If they survive to sexual maturity, and ideally after they have successfully reproduced, most wild animals succumb to disease or predators. Because we humans have made advances against disease, and because we have no natural predators, other than arguably ourselves, we actually age, a phenomenon rarely seen in the wild. Of course, animals brought into the laboratory and subjected to carefully controlled conditions can also survive long enough to age, providing a unique opportunity for study.

To age successfully, with a minimum of disease, discomfort, or disability, we must understand the processes that cause aging. The oldest confirmed human lifespan is 122 years (Jeanne Calment of France). Today, scientists are trying to understand what has thus far prevented all but a handful of people from living into their 12^th and 13^th decades. If they can understand the causes of age-related disease and early death, perhaps they can enable the current maximum life span to become the norm.

Theories of aging are almost as old as human civilization. The earliest scientists, who were really philosophers, argued that we are born with predetermined amounts of some vital substance. Once that vital substance is consumed in the course of living, we die. Some modern theories of aging have grown out of this and other early ideas. Still other theories reflect our ever-expanding understanding of the human body and its inner workings.