We have introduced what evolutionary biologists think about the evolution of aging. Today, it is clear that aging is not a positively selected, programmed death process, and has not evolved for "the good of the species". Instead, aging is a feature of life that exists because selection is weak and ineffective at maintaining survival, reproduction, and somatic repair at old age. Based on the observation that the force of selection declines as a function of age, two main hypotheses have been formulated to explain why organisms grow old and die: the mutation accumulation (MA) and the antagonistic pleiotropy (AP) hypotheses. Under MA, aging evolves because selection cannot efficiently eliminate deleterious mutations that manifest themselves only late in life. Under AP, aging evolves as a maladaptive byproduct of selection for increased fitness early in life, with the beneficial early-life effects being genetically coupled to deleterious late-life effects that cause aging. Aging clearly shortens lifespan, but lifespan is also shaped by selection for an increased number of lifetime reproductive events. The evolution of lifespan is therefore a balance between selective factors that extend the reproductive period and components of intrinsic mortality that shorten it. Whether there exist truly immortal organisms is controversial, and recent evidence suggests in fact that aging might be an inevitable property of all cellular life.