Although most gerontologists, sometimes unconsciously, are ready to admit that ageing and life-span are genetically determined, it is a paradox to recognize that for the last 25 years most theories of ageing were stochastic in nature. The evidence in favour of such theories is reviewed and it is shown that they are most probably wrong. Yet the evidence in favour of a genetic basis for ageing and life-span is scarce and, until now, no definite proof of such a determinism has been provided.