Projections of duration of life for humansbased on mathematical models have led someresearchers to claim that there is no lowerlimit to death rates or upper limit to lifeexpectancy, and that a life expectancy of 100will be achieved in the 21st century. Toassess the biological plausibility of theseclaims, we examined temporal aspects ofbiological phenomena in three mammalianspecies. Our examination revealed that: (1)physiological declines associated withreproduction consistently occur at ages thatare less than one-third of the median age atdeath, (2) physiological parameters associatedwith aging in humans lose eighty percent oftheir functional capacity by age 80, and (3)young versus old individuals can bedistinguished by the pathologies detected atdeath. The biological evidence suggests thatorganisms operate under warranty periods thatlimit the duration of life of individuals andthe life expectancy of populations. We usethese findings to discuss the issue of limitsto the duration of life and the validity ofmathematical models used to forecast humanlongevity.