A fact that has been repeatedly observed over the last 200 years is that women live longer than men. Therefore, this difference in longevity cannot be solely accounted for by the different sociocultural roles played by each of the sexes in the core of a community since this gap remains consistent in societies that greatly differ in their customs, settlement locations, and lifestyles. Thus, over several decades an effort has been made to determine whether there is a biological mechanism that could explain this difference. This research has produced a wide range of results that could be summarized in four main hypotheses based on genetic disparities, the role of sex hormones, immune system dimorphism, and body fat distribution. However, determining which one of these hypotheses plays the most decisive role in the biology of the gender longevity gap has proven quite an arduous task. Moreover, these hypotheses are not mutually exclusive; instead, they link the differences in lifestyle behaviour of men and women in an attempt to elucidate the reason why women live longer than men, a highly peculiar and remarkable fact of human existence.