New data has challenged the convention that the adult Sertoli cell population is fixed and unmodifiable. The Sertoli cell has 2 distinct functions, i) formation of the seminiferous cords and ii) provision of nutritional and structural support to developing germ cells. For these to occur successfully, Sertoli cells must undergo many maturational changes between fetal and adult life, the main switches occurring around puberty, including the loss of proliferative activity and the formation of the blood testis barrier. Follicle stimulating hormone plays a key role in promoting Sertoli cell proliferation while thyroid hormone inhibits proliferative activity in early postnatal life. Together these regulate the Sertoli-germ cell complement and sperm output in adulthood. By puberty, the Sertoli cell population is considered to be stable and unmodifiable by hormones. But there is mounting evidence that the size of the adult Sertoli cell population and its maturational status is modifiable by hormones and that Sertoli cells can gain proliferative ability in the spermatogenically disrupted hamster and human model. This new information demonstrates that the adult Sertoli cell population at least in the settings of testicular regression in the hamster and impaired fertility in humans in vivo and from mice and men in vitro is not a terminally differentiated population. Data from the hamster now shows that the adult Sertoli cell population size is regulated by hormones. This creates exciting prospects for basic and clinical research in testis biology. The potential to replenish an adult Sertoli-germ cell compliment to normal in a setting of infertility may now be realised.