Sex differences in lifespan have been labelled as one of the most robust features in biology. In human populations, women live consistently longer than men, a pattern that encompasses most mammalian species. However, when expanding both the taxonomic scope beyond mammals and the range of mortality metrics the female survival advantage over males is no longer the rule. Moreover, current evidence suggests that sex differences in actuarial ageing parameters (i.e. age at the onset of ageing and rate of ageing) are far from consistent across the tree of life. This chapter first reviews current knowledge of sex differences in mortality patterns across animals and appraises how these diverse patterns can be explained by the current evolutionary framework. It then emphasizes the relevance of going beyond the differences in mortality patterns by exploring how natural and sexual selection have shaped age- and sex-specific changes in reproductive performance and body mass across the tree of life, and by identifying some possible biological pathways modulating ageing in a sex-specific way. Finally, it highlights how evolutionary theories can be relevant to understand the widespread differences in causes of death between sexes, offering a complementary approach to gain a comprehensive understanding of the evolution of sex differences in health and ageing, with likely biomedical implications.