Humans age. Domestic animals age. But is that true for all species? Is ageing a necessary consequence of evolution? Yes - for a long time, this was the undisputed answer of classic evolutionary theories of ageing. This chapter tells the story about how this paradigm of inevitable ageing has been challenged and refuted. Thanks to decades of monitoring individual survival and death across species in captivity and in the wild, researchers have been able to study patterns of the ageing process’s ultimate consequence - age trajectories of mortality. Though ageing is a complex, multiscale process, increasing mortality with age is, overall, indicative of a loss of functioning with age - senescence. Constant or declining mortality with adult age is indicative of maintained or improved functioning - negligible or negative senescence. Evidence supports that ageing patterns across the tree of life are diverse. Whether current evidence for negligible or negative senescence truly reflects an absence of senescence or just an absence of evidence is an open challenge. Similarly, why certain types of species show certain types of senescence patterns is an open research question. Future evolutionary theories of ageing will have to include trade-offs justified by structural arguments - genetic structure, physiological structure, social structure, ecological structure - to explain types of ageing patterns across types of species.