The dream of eternal youth and immortality has always fascinated human societies. Even today, this quest is the source of major financial investments, particularly for the development of anti-ageing drugs. To unravel the mysteries of longevity, scientists have long been observing and quantifying the lifespan of animals. These decades of extensive comparative biology research have documented the extreme diversity of lifespan on Earth and identified key ecological and life history factors driving this diversity and, more recently, molecular pathways that might modulate it. However, the maximum lifespan of a species is far from being an accurate representation of a species’ ageing trajectory, both biologically and demographically. For a given species, the changes in mortality risk over the life course can be complex, and the ageing process is much more accurately described by ageing parameters, such as the age of onset of actuarial senescence and the rate of actuarial senescence. This chapter argues that current research in the comparative biology of ageing should now focus on the diversity of actuarial senescence patterns documented across the tree of life, as well as the species-specific causes of death, to identify key genetic and physiological determinants associated with delayed actuarial senescence or low actuarial senescence rate. Just a few years ago, such research projects would have seemed unrealistic, but the recent development of omics tools, coupled with the increased availability of demographic data for a wide range of species with contrasting life histories, lifestyles and habitats make such exciting comparative analyses now achievable and full of promise.