Evolutionary biologists have shown mathematically that aging is an inevitable consequence of age-specific natural selection acting on species with somata separate from germ lines. Two specific genetic mechanisms are known which could underlie the evolution of aging under these conditions: age-specificity of gene effects and antagonistic pleiotropy between early and late ages. Comparative evidence indicates that senescence occurs only when the stipulations of the evolutionary theory are met. Laboratory experiments with Drosophila indicate that prolonging the action of natural selection leads to the evolution of postponed senescence. The genetic variation involved in such postponed senescence exhibits both age-specificity and antagonistic pleiotropy. These theories and empirical findings together suggest that the best general theory of aging now available is the evolutionary theory. In addition, this work has yielded Drosophila stocks with postponed senescence that are being used to unravel physiological mechanisms of senescence.