Senescence, the endogenous deterioration of health at later ages, can be explained in terms of evolution. Senescence is not due to group selection but to the decline with age in the force of natural selection acting on individuals. This decline allows the spread of alleles with deleterious effects on late health. Such alleles do not appear to have effects confined to later ages. Instead, they are favoured by natural selection because of beneficial effects at early ages, in spite of later deleterious effects due to antagonistic pleiotropy. Manipulation of laboratory populations of Drosophila has shown that senescence can be postponed using selection. There are no absolute, universal, physiological causes of senescence. Laboratory populations with genetically postponed senescence can be used to study proximate physiological mechanisms of senescence in animals.