Analysis of recent data on telomeres and telomerases, limitations of proliferative potential of untransformed cells (Hayflick's limit), kinetics and molecular mechanism of cell proliferation and differentiation, and factors that determine the longevity of multicellular organisms suggests that: (i) chromosome end shortening during consecutive divisions of cells that lack telomerase limits cell proliferation beyond Hayflick's limit; (ii) Hayflick's limit can be a manifestation of mechanisms that determine the size of the living organism (it correlates with longevity) but not the longevity of multicellular organisms.