Spermatogenesis is regulated by hormones. Pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH) act on testicular Sertoli cells and Leydig cells, respectively, to stimulate their proliferation and function. Leydig cell-derived testosterone stimulates and maintains spermatogenesis by acting on Sertoli cells together with FSH. Without testosterone, spermatogenesis is arrested in meiosis and only few haploid spermatids develop, unless the man has an activating FSH receptor (R) mutation. Men with inactivating FSHR mutations produce only low numbers of sperm. Depending on remaining functional activity, androgen receptor defects lead to azoospermia or oligozoospermia. Androgen receptors are expressed only in the somatic cells of the testis, and therefore, androgen-dependent germ cell development is governed by the cooperation of Leydig cells, peritubular myoid cells, and Sertoli cells.