Human Leydig Cells (LC) are located in the interstitium of the testis near the seminiferous tubules. One of the main functions of LC is to produce testosterone after stimulation by pituitary luteinizing hormone. LC, through synthesis of testosterone, play a key role in mammalian sex determination by inducing differentiation of the Wolffian ducts into male reproductive organs. Additionally, LC produce insulin-like growth factor 3 which is necessary for scrotal descent of the testis. Recent studies indicates that molecules such as cyclic nucleotide phosphodiesterases can modulate testosterone production in LC; suggesting that pharmacological targeting may manipulate testosterone synthesis. Cultured Human LC are a useful in vitro model to better understand testicular development and also to develop treatments for male reproductive disorders. They are also useful tools to stablish in vitro disease models for High Throughput and High Content Screening.