Mammalian testes continually produce a huge number of sperm over a long reproductive period. This constant spermatogenesis is supported by a highly robust stem cell system. Morphological analyses in the 1960s and 70s established the basis of mammalian spermatogenesis and the associated stem cell research. Subsequently, from the 1990s on, functional analyses, which have included post-transplantation colony formation, in vitro spermatogonial culture with persisting stem cell activity, in vivo lineage tracing, and live imaging, and also lines of molecular-genetic analyses, have contributed greatly to our understanding of mammalian spermatogenic stem cells. This review will provide a brief overview of the history of this field and then go on to describe in detail the progress made in recent years.