The problem with long life is that one keeps getting older; here's an able and clearly written summary of the latest theories on why we age and what might be done to ameliorate the process. Austad, a biologist at the universities of Idaho and Washington and science advisor to NPR, begins with an examination of longevity. Despite anecdotal claims of ages in excess of 150 years, modern medicine has concluded that there is no evidence of a human living much past the age of 120. While the human life span has unquestionably undergone a dramatic increase during this century, almost all the gain has come in the elimination of infectious diseases, especially those of childhood. If a complete cure for cancer were discovered tomorrow, it would add at most a couple of years to the average life expectancy. Austad notes other fascinating patterns, such as a huge leap in the male death rate during the period of ""testosterone dementia"": adolescence and early maturity. The discussion then turns to the biological mechanisms of aging. Among the explanations that have become current at various times is the notion that aging and death are evolutionary mechanisms for removing obsolete stock from the gene pool. Another theory is that there is an arbitrary limit on the number of times a given cell can divide; Austad refutes these and other theories. Especially interesting is his examination of the relation between menopause and aging, and the use of hormone therapies to inhibit aging in older women. Finally, he turns to the current theories, scientific and otherwise, on therapies to postpone aging: reduced calorie intake, exercise, and such trendy nostrums as melatonin. While he is skeptical of many of the claims for such anti-aging therapies, he remains optimistic that continued research may enable our descendants to look forward to a longer and healthier lifespan than we can.