Senescence is not limited to humans and laboratory organisms. In the wild, there is compelling evidence that both age-specific survival and fecundity decrease with increasing age. While these senescence patterns are highly variable across species and populations, it is now clear that the process of senescence is the rule rather than the exception in the living world. For a long time, the occurrence of senescence was considered as puzzling because a process with a negative influence on fitness should be counter-selected by natural selection. In this article, we perform a global assessment of the most influential theories that have been proposed to solve this paradox. Then, we evaluate whether the recent progress made in identifying the physiological basis of senescence in field studies fit the current theoretical framework proposed to explain the evolution of senescence in the living world.