Two recent major developments in the molecular biology of lower organisms point to genes as determinants of longevity. The first line of evidence is a selection system that established the participation of genes in aging and subsequently showed longevity to be a polygenic characteristic in Drosophila. The second validation of a genetic role in aging came from studies in which a mutation in an individual gene was found to modulate life span in nematodes. The achievements of genetics in the analysis of aging in mammals are less impressive than they have been for lower organisms, but this situation is changing. Although no genes have yet been directly implicated in prolonging life span in mammals, studies with mice have related immune function to longevity.