Any modern theory of longevity has to take into account the multifactorial combined effect of biological-medical, social and psychological factors and of factors pertaining to molecular biology, each with their different weighting. For this reason research into greater-than-average life expectancy and longevity is only possible on an interdisciplinary basis. Many theories of longevity despite convincing arguments also have their weak points. Critical analysis of the theories available to date reveals that longevity rests on an optimal combination of two major factors: 1. on a genetic predisposition in people with longevity in the family--this is the case in most instances. 2. on additional exogenous factors of a social, psychological, ecological and medical kind.