Spermatogenesis is the process of male germ cell proliferation and differentiation that begins at puberty and lasts throughout life. Spermatogonia, especially stem spermatogonia, are the cells essential for the continued maintenance of spermatogenesis. Although many studies of spermatogonia have been performed with morphological methods, the very nature of spermatogonia still remains unknown. The technique of spermatogonial transplantation, developed in 1994, made it possible to study functional aspect of spermatogonial stem cells. Many new developments, such as cryopreservation, xenotranplantation, purification, and culturing of spermatogonial stem cells, have been achieved and are still under investigation. The techniques could be used not only for basic research but also for medicine and other disciplines.