The spermatogonial transplantation technique was developed by Dr. Ralph Brinster in 1994. Transplanted spermatogonial stem cells (SSCs) produce germ cell colonies after microinjection into the seminiferous tubules of infertile mice. This technique provided the first functional assay for SSCs. Although it became possible to produce transgenic animals using this transplantation technique in 2001, the lack of SSC culture systems prevented efficient genetic manipulation. To overcome this problem, a long-term SSC culture technique was developed in 2003. Cultured SSCs, designated as germline stem cells, allow drug selection of transfected SSCs, and knockout mice were produced in 2006. Using these techniques, it is now possible to address basic biological questions of SSC biology. They also open up new possibilities for male germline manipulation. In this review, we will briefly summarize our findings on SSCs and discuss unresolved issues that remain to be addressed.