Spermatogonial stem cells (SSCs) are responsible for the continual production of spermatozoa throughout adult life. Interactions between SSCs and the surrounding cells in the seminiferous tubules regulate the biological activity of these cells. Factors involved in the regulation of SSCs are beginning to be defined by animal models and the culture of SSCs in defined media. A critical development in the characterization of SSCs has been the development of the germ cell transplantation technique, which provides the only assay for the presence of SSCs in a population of cells, and which allows the determination of whether SSCs are proliferating or differentiating in culture. This approach has accelerated SSC-focused research and promises to provide a better understanding of the factors and mechanisms that regulate these cells. The knowledge provided by this work is also critical to an appraisal of the components of the SSC niche in the seminiferous epithelium. Thus, many aspects of testicular function can be defined by the investigation of SSCs and the factors, cells, and environment that regulate SSCs, thereby leading to a more comprehensive understanding of spermatogenesis.