Mouse spermatogenesis represents a highly potent and robust stem cell system. Decades of research have made it one of the most intensively studied mammalian tissue stem cell systems. These studies include detailed morphological examinations, posttransplantation colony formation, and in vitro culture of the stem cells; however, the nature of the stem cells as well as their niche are mostly to be elucidated in the context of homeostatic spermatogenesis. Our group has been challenging this issue by means of transgenic and live-imaging approaches that enable the investigation of live behaviors of "undifferentiated spermatogonia," the candidate stem cell population. A pulse-label experiment has suggested a hierarchical composition of the stem cell functional compartments, unlike the general idea. In addition, live imaging revealed the preferential localization of undifferentiated spermatogonia in the area adjacent to the blood vessel, leading to the proposal of a vasculature-associated niche. These results have suggested the idea of "flexibility" in the mouse spermatogenic stem cell system, which makes a good contrast to the "strict" stem-cell-niche system observed, for example, in the Drosophila germ line. This flexible nature seems to be advantageous for mammalians.