Pioneering studies suggested the existence of a specific receptor for LH (Lhr/LHCGR) in Leydig cells based on LH-induced testicular testosterone production. This was further supported by the specific high-affinity binding of radiolabeled hCG (a strong LH agonist) to Leydig cells. A plethora of information on the physiological function and regulation of the putative Lhr was subsequently obtained through ligand binding studies. The next milestone was the cloning of the Lhr gene, which opened up access to its molecular and structural biology, including detailed molecular mechanisms of its action. The purpose of this chapter is to briefly review the history of Lhr studies, concentrating on key findings on its role in the regulation of Leydig cell function in humans and experimental rodent models.