The effect of ageing on growth, function and cell loss in differentiated cells is reviewed briefly. Although there is some evidence of loss of function and of delay in the initiation of cell division in ageing cells, there is no evidence of massive cell loss. It is unlikely that an overall loss of cells or cell malfunction is responsible for limitation of total life-span of an individual. Although there seems to be a programmed life-span for some cells analogous to the 40–50 cell division cycles found in some tissue culture experiments, few normal mammalian cells undergo their total possible number of cell divisions. It is suggested that most of the widely varied age-associated changes which are found are due to random damage and that the total possible life-span of the individual may be controlled by small groups of essential cells in specific organs and tissues. The life-span of these cells could be programmed during development as part of the normal process of differentiatio