INTRODUCTION UNDERSTANDING the nature of the primary events leading to ovarian androgen production is one of the major goals of reproductive research. The physiological importance of this research is emphasized by the fact that ovarian androgens are obligatory precursors for estrogen biosynthesis (1, 2) and they induce follicle atresia (3, 4). Obviously, any disturbance in ovarian androgen metabolism will profoundly affect the reproductive state of the female. Indeed, many kinds of reproductive abnormalities in women are caused by disturbances of ovarian androgens (5–7), and therefore, knowledge and understanding of androgen processes in the ovary are important to human welfare. Over the past several years, a series of studies in our laboratory has provided important new information about the underlying controlling mechanisms governing ovarian androgen biosynthesis. The most important event that led to these new observations was the development of a primary cell culture model in which the processes of hormone dependent differentiation of ovarian interstitial cells can be studied in serum-free defined medium (8, 9). This article reviews some of the findings and their implications for understanding ovarian function. We will first introduce the subject of ovarian interstitial cells and discuss a few problems of developmental interest. We will then discuss what the concepts and principles of the interstitial cells are, how they came about, and how they provide knowledge applicable to ovarian physiology and pathophysiology.