Cellular research in aging has been stimulated by the observation that human diploid cells have a limited number of cell divisions in culture. This loss of cellular proliferation (in vitro senescence) has been extensively studied by biochemical, clonal, and genetic analysis. Studies of human skin fibroblast cultures have revealed that in vitro senescense is related to in vivo human cellular aging. Recently differentiated cells have been proposed for aging studies. These cells may provide additional information on aging since alterations of in vitro cellular functions may be related to the in vivo behavior of specific differented cell types.