The origin and development of the spermatogenic cell lineage is reviewed, as well as spermatogonial kinetics in adult nonprimate mammals in relation to the cycle of the seminiferous epithelium, the emphasis being on spermatogonial stem cells. A hypothesis is presented for the transition from foetal germ cells, gonocytes, to adult type spermatogonia at the start of spermatogenesis. An overview is given of the present knowledge on the proliferation and differentiation of undifferentiated spermatogonia (spermatogonial stem cells and their direct descendants) and the regulation of these processes. It is concluded that the differentiation of the undifferentiated into differentiating type spermatogonia is a rather vulnerable moment during spermatogenesis and the models for studying this are described. Research into the molecular basis of the regulation of spermatogonial proliferation, differentiation and apoptosis is at its infancy and the first results are reviewed. An exciting new research tool is the spermatogonial stem cell transplantation technique which is described. Finally, reviewing the nature of human germ cell tumours it is concluded that at present there are no animal or in vitro models to study these tumours experimentally.