Life span is subject to genetic modification in yeasts, nematodes, fruit flies, mice, humans, and other vertebrates and invertebrates. There are a few single-gene mutants known that extend life span in yeast and nematodes; in other experimental systems the character is treated quantitatively, and generally has a low to moderate heritability. Life span responds to artificial selection in Drosophila and Caenorhabditis. There are many candidate genes presently under investigation, including the anti-oxidizing enzymes and heat-shock proteins. The main evolutionary models of senescence are antagonistic pleiotropy and mutation accumulation, neither of which has substantial experimental support. The incorporation of analytical techniques from demography is playing an increasing role in research on aging.