This chapter provides an update on spermatogonial stem cells (SSCs), on their role in male fertility, and on (future) clinical applications using these fascinating cells. Spermatogenic proliferation and differentiation is accompanied by incomplete cell division, resulting in daughter cells, which remain interconnected by intercellular bridges. Spermatogonial stem cells can develop in three different ways: they can renew themselves, they can differentiate, or they can go into apoptosis. To safeguard the reproductive potential of young cancer patients, cryopreservation of testicular tissue containing SSCs is preferred above cryopreservation of SSC suspensions. The technique of spermatogonial stem-cell transplantation involves the introduction of a germ-cell suspension from a fertile donor testis into the seminiferous tubules of an infertile recipient mouse. Testicular tissue grating has been suggested as an alternative to spermatogonial stem-cell transplantation. Banking and transplantation of SSCs may become a promising method to preserve the fertility of prepubertal patients.