Historically, women have lived longer than men in almost every country in the world. A similar pattern of sex differences in longevity is also found in many other species; however, it is not clear if there are more species in which females live longer or vice versa. For virtually all the primary causes of death and at virtually all ages, mortality rates are higher for men. Women do not live longer than men because they age more slowly, but because they are more robust at every age. Paradoxically, although women have lower mortality rates they have higher overall rates of physical illness than do men. Several hypotheses have been proposed for sex differences in longevity, including more active female immune functioning, the protective effect of estrogen, compensatory effects of the second X chromosome, reduction in the activity of growth hormone and the insulin-like growth factor 1 signaling cascade, and the influence of oxidative stress on aging and disease. At present, none of these hypotheses are strongly supported, although weak support is available for the oxidative stress hypothesis. With the advent of more rapid genome sequencing, molecular tools will become available for more species, thus further detailing the causes for the differences in longevity between the sexes.