Sex differences in ageing are ubiquitous across the tree of life. Several hypotheses have been put forward to explain this phenomenon, based on both adaptive processes, invoking sex-specific selection pressures, and maladaptive processes, based on the asymmetric inheritance of mitochondrial DNA and sex chromosomes. In the first part of my thesis I used Drosophila melanogaster to address a series of open questions on how adaptive processes may shape sex-specific ageing. To begin with, I studied whether social context can modulate age effects on female/male reproductive success and found that it did not modulate age‐related fitness decline in either of the two sexes. Second, I investigated how condition-dependent mortality (i.e. simulated predation) affects female/male reproductive ageing. Although female reproductive ageing was not affected by predation, male reproductive ageing was considerably higher under predation, due mainly to an accelerated decline in the viability of ‘surviving’ males’ offspring with age. Third, I explored the mechanisms affecting male life history by studying the relationship between gut microbiota composition and life history traits in males. Our results suggest that the genus Acetobacter is particularly associated with ageing in D. melanogaster males. In the second part of the thesis, I tackled the two main maladaptive hypotheses put forward to explain widespread patterns of sex-specific ageing across taxa. The “unguarded-X” hypothesis (UXh) explains sex-specific ageing by differential expression of recessive mutations in the X/Z chromosome of the heterogametic sex (e.g., females in birds and males in mammals). A recent complementary hypothesis, the “toxic Y”, suggests that the accumulation of deleterious mutations and repetitive elements on the Y/W chromosome might lower the lifespan of the heterogametic sex across the tree of life. First, I used D. melanogaster to test and confirm a central prediction of the UXh: that inbreeding decreases the lifespan of the homogametic sex more than the heterogametic sex; only in the former does inbreeding increase the expression of recessive deleterious mutations. Second, I conducted a comparative meta-analysis across vertebrates and found lower survival of the heterogametic (relative to the homogametic) sex across 138 species of birds, mammals, reptiles and amphibians, as expected if sex chromosomes shape sex-specific lifespans. Moreover, the size of the Y chromosome correlates negatively with male survival in mammals, supporting the “toxic Y” hypothesis. In this PhD I have hopefully shed some light into open questions about adaptive and maladaptive processes of life history evolution, and hence to our general understanding of sex-specific ageing.