The objectives of this review were first to critically review what is known about the effects of aging on stem cells in general, and hematopoietic stem cells in particular. Secondly, evidence is marshalled in support of the hypothesis that aging stem cells play a critical role in determining the effects of aging on organ function, and ultimately on the lifespan of a mammal. Aging has both quantitative and qualitative effects on stem cells. On balance, the qualitative changes are the more important since they affect the self-renewal potential, developmental potential, and interactions with extrinsic signals, including those from stroma. Although hematopoiesis is generally maintained at normal and life-supporting levels during normal aging, diminished function is acutely apparent when old stem cells are subjected to stress. There is ample evidence of diminished self-renewal capacity, restriction of the breadth of developmental potency, and decreased numbers of progeny of old stem cells subjected to hematopoietic demands. The prediction is made that whatever plasticity in developmental potential possessed by a young stem cell is lost during aging. Those parts of the world enjoying an ever-increasing standard of living are also inhabited by an increasingly elderly population. The effects of age on many physiological functions are not well studied or appreciated. A public health challenge to provide increased quality of life for this growing segment of the population requires more attention to the variable of age in experimental studies. Stem cell populations are likely to be a fruitful subject for studies of this type.