Spermatogenesis, the process essential for male fertility, relies on the continuous supply of differentiating germ cells from a pool of spermatogonial stem cells. Spermatogenesis is dependent upon hormonal stimuli that result in a complex pattern of intratesticular signalling pathways. The two main hormones responsible for its control are pituitary follicle stimulating hormone (FSH) and testicular testosterone. Testosterone and FSH act through their somatic cell receptors in the testis to promote the initiation of spermatogenesis, increase in germ cell numbers, development and differentiation. This knowledge has been advanced by animal models, but these models often pose more questions than they answer. However, when all the evidence provided by these studies is taken together, some important conclusions can be drawn. Evidence from animal models confirms that testosterone is required for completion of meiosis and spermiation, and can have a stimulatory effect on spermatogonia numbers in both rodent and primate models of gonadotropin suppression, even in the absence of FSH. FSH alone cannot drive spermatogenesis to completion, but it has a stimulatory effect on both Sertoli cell number and spermatogonia number that results in a higher adult sperm output. Evidence suggests that testosterone and FSH are more important for the development of spermatogonia in primates than they are in rodents. The sum of this knowledge highlights future studies that are required to develop male contraceptives or infertility treatments.