Spermatogonial stem cells have the remarkable capacity to regenerate spermatogenesis in infertile testes. Data from several animal models, including mice, rats, pigs, goats, dogs and rhesus monkeys demonstrates that transplantation of SSCs into the testes of infertile males can lead to restoration of spermatogenesis. In addition, SSCs from males at all postnatal ages, newborn to adult, are competent to produce complete spermatogenesis following transplantation into recipient testes. In addition, we know that SSCs which are cryopreserved and later thawed retain their ability to regenerate spermatogensis upon transplantation. Therefore, SSCs are an excellent model to study the process of regeneration from an adult tissue stem cell in mammals. We use SSC transplantation as an functional endpoint to study the fundamental regulation of the regenerative process in spermatogensis.