Elucidating the regulation of androgen biosynthesis in ovarian theca cells is not only important for determining the mechanisms of regulation of estrogen biosynthesis throughout the menstrual cycle, but is also essential for understanding the pathogenesis of excess androgen biosynthesis and polycystic ovary syndrome (PCOS). Human theca cells in primary and long-term culture have provided model systems for examining theca cell differentiation as well as the mechanisms underlying basal and cAMP-regulated steroid biosynthesis at both the transcriptional and post-transcriptional level in normal and PCOS ovaries. Results of these studies are expected to lead to the identification of novel targets for clinical treatment of infertility and PCOS.