This review discusses how recent methodological developments that have facilitated study of spermatogonial stem cells (SSCs) in vitro and in vivo have been used in conjunction with knockout mice to gain important insights into the factors that regulate SSC maintenance, self-renewal and differentiation. We first consider the critical role of the GDNF/RET/GFRα signaling pathway in regulating SSCs. We then subsequently focus on what has been learned about SSC maintenance from a newly developed knockout mouse lacking the transcription factor ets-related molecule (ERM). ERM is necessary for maintenance of spermatogonial stem cells (SSC) in the testis; in the absence of ERM, the first wave of spermatogenesis occurs normally, but all SSCs differentiate and are lost during this time. This is the first time a transcription factor has been shown to be essential for stem cell self-renewal. Understanding how ERM, the GDNF/RET/GFRα signaling pathway and other factors act to maintain SSCs will advance our understanding of how these cells, and stem cells in general, are regulated. This work has potential clinical implications for human and animal infertility, and may also provide novel contraceptive targets and strategies.