I. Introduction DURING the last decade, it has become apparent that the pituitary gonadotropins, FSH and LH, are not the exclusive regulators of ovarian function. Several other hormones and various growth factors also play a role in ovarian physiology. Among these hormones and growth factors are insulin and insulin-like growth factors (IGFs). It is conceivable that a deficiency or an excess of insulin or IGFs could significantly alter ovarian function. In fact, insulin-dependent diabetes mellitus (IDDM), a disorder characterized by insulinopenia, is associated with clinical manifestations of ovarian hypofunction: primary amenorrhea, late menarche, anovulation, low pregnancy rate, and early menopause. Furthermore, insulin resistance, the cardinal feature of which is hyperinsulinemia, is associated with clinical manifestations of ovarian hyperstimulation, primarily hyperandrogenism. It is becoming increasingly clear that insulin plays an important role in ovarian physiology; however, the mechanisms of insulin action on the ovary remain largely hypothetical. We will review clinical observations and experimental studies that shed light on the role of insulin in ovarian function.