This monograph, Senescence; Dominant or Recessive In Somatic Cell Crosses? represents the second annual workshop to promote theory and concept development in aging research. These workshops are part of a resource to bank cultured cells of special interest to aging research that was established at the Institute for Medical Research in Camden. New Jersey. by the National Institute on Aging in 1974. The underlying theme of the workshops is the use of cultured cells in a variety of somatic cell genetic systems designed to define mechanisms of in vitra cellular scen­ escence and the possible insights that this may provide to the problems of in viva aging. The concept also includes bringing together workers from a variety of disciplines to stimulate new and innovative thoughts and work in the area. The current work­ shop focuses on the relative role of nucleus and cytoplasm on determining the in vitra lifespan of human diploid cells as well as the relative influence of old and young cells when combined within a single cell structure. The techniques and procedures discussed should make significant contributions to understanding in vitra senescence and may lead to the mapping of an area or areas of the genome linked to senescence as is being accomplished with viral transformation of normal cells. Warren W. Nichols Donald G. Murphy ~i Contents Theoretic Mechanisms of in vitpo Senescence 1 F. MaPott Sinex . . . . . . . . . . . . Senescence in Ce1l Cu1ture: An Accumu1ation of Errors or Terminal Differentiation? 13 Vincent J. GPistofaZo . . . . . . . . . . . . . .