All stages of male reproductive development and function are highly regulated, not only by the gonadotropins FSH, LH, and testosterone, but also by local control signals regulating cell function, mitosis, meiosis, and differentiation. The present approach to the treatment of many types of male infertility focuses on the use of suboptimal or defective sperm with protocols designed to overcome specific abnormalities. Elucidation of the molecular events required for normal male reproduction using animal model systems will eventually permit the design of new diagnostic procedures and therapeutic modalities. These advances may lead to enhancement or restoration of fertility in previously sterile men.