Cellular or replicative senescence is classically seen as the key element of aging. In renal disease and after kidney transplantation, there is increasing evidence that replicative senescence pathways (p53 and p16) play a central role in disease progression and graft outcome, independent of chronological age. In this review, we summarize the current concepts in the molecular mechanisms of cellular senescence, and correlate these theories with the available literature on aging of native kidneys, kidney diseases, and outcome of renal allografts. Recent data illustrate the complex biology of senescence in vivo, and disprove the concept that senescence is an intrinsic injury process with immanent deleterious consequences. Senescence acts as a homeostatic mechanism that can even limit renal fibrosis, at least in animal studies. In a human setting, it remains to be investigated whether cellular senescence plays an active or a bystander role in fibrogenesis and atrophy of renal tissue.