(A) There are data showing beyond question a gender gap, with women living longer than men, especially in economically developed societies. There is greater male vulnerability to the major causes of human death. (B) In lower animals there are data suggesting a female survival advantage to adult life in many species, but the observations do not consider longevity or survival to an advanced age. (C) In laboratory rodents kept under controlled conditions the relationship of sex to longevity is variable, with males sometimes showing greater longevity than females and with life span being dependent on factors like breeding and diet. (D) Similar genetic and hormonal processes operate in humans and in non-human mammals including the genetic mechanism of sex determination, the hormonal consequences of sex determination, and the effects of hormones on processes which affect longevity such as cholesterol levels and immune functions. (E) Causes of death in humans and animals are different, and it seems unlikely, therefore, that the same mechanisms could be determinants of longevity in all mammalian species. (F) Human male and female longevity continue to change, and it is likely that the gender gap will narrow, with societal and medical changes in post-industrial societies reducing the male disadvantages in behaviour and in the handling of cholesterol. (G) It remains an important question whether part of the gender gap seen in humans is based on other differences in the basic biology of males and females.