Spermatogonial stem cells (SSCs) are essential for long-term spermatogenesis and are the subject of considerable clinical interest, as 'SSC therapy' has the potential to cure some forms of male infertility. Recently, we have learned more about SSCs and spermatogenesis in general from a plethora of studies that performed single-cell RNA sequencing (scRNAseq) analysis on dissociated cells from human, macaque, and/or mice testes. Here, we discuss what scRNAseq analysis has revealed about SSC precursor cells, the initial generation of SSCs during perinatal development, and their heterogeneity once established. scRNAseq studies have also uncovered unexpected heterogeneity of the larger class of cells that includes SSCs - undifferentiated spermatogonia. This raises the controversial possibility that multiple SSC subsets exist, which has implications for mechanisms underlying spermatogenesis and future SSC therapeutic approaches.