The kinetics of spermatogenesis in mammals are reviewed with special emphasis on the seminiferous epithelium cycle and spermatogonial renewal . 2 features of the spermatogenic process are that for a given species each step of spermatogenesis has a constant duration and that many spermatogonial stem cells enter spermatogenesis simultaneously and almost synchronously. The seminiferous epithelium composed of 5 or 6 generations of germ cells form cellular associations of fixed compositi on. Spermatids at a given step of spermiogenesis are always associated with spermatocytes and spermatogonia at given steps of development. These cellular groupings were taken to represent stages of a cycle of the seminferous epithelium. The number of stages composing the cycle vary with the species and the criteria used to characterize them. The classifications of cell associations are particulary useful in investigations of the mode of renewal of the spermatogonial population. The stem cells that belong to the Type A category may be classified as "reserve" and "renewing" stem cells. The former appear to be actively involved in the production of spermatocytes in normal adult animals but the latter initiate at each cycle of the seminiferous epithelium a series of mitoses during which they renew themselves and simultaneously give rise to "differentiating" spermatogonia. The "nodal" or key mitosis that result appear to be equivalent in type and not bivalent.