which occur during normal folliculogenesis. Follicles grow through two stages, the rst being gonadotropin-independent and the second gonadotropin- dependent. Primordial follicles consist of an oocyte with a thin layer of granulosa and stromal cells, and at this stage, follicles can­not be seen by ultrasound. By the time follicles develop an antral cavity, they become ultrasono­graphically visible, and, importantly, they have reached the gonadotropin-dependent stage. These antral follicles measure between 2 and 10mm in mean diameter and represent the pool of follicles that may be stimulated in the ensuing follicular phase. In a natural cycle, one follicle is ultimately selected for ovulation, and that selection process occurs during the mid-follicular phase of the ovarian cycle, when the endogenous pituitary fol­licle stimulating hormone (FSH) level falls in response to the increasing ovarian estradiol pro­duction. Decreasing FSH levels promote a selec­tion process in which each of the follicular microenvironments competes for the diminishing FSH needed to stimulate granulosa cells to pro­duce aromatase and continue growth. Aromatase, in turn, is necessary to convert testosterone and androstenedione produced in the peripheral theca cells into estradiol and estrone, respectively, in the granulosa cells. Failure of this conversion from androgen to estrogen leads to an elevated androgen-to-estrogen ratio in the microenviron­ment of the follicle and then to follicular atresia. From this cursory review of the anatomy and physiology of oocyte maturation, it is easy to see how ovulation-inducing agents that either indi­rectly increase endogenous FSH (e.g., clomi­phene citrate) or directly add exogenous FSH to the system diminish the competition among fol­licles for FSH and permit the development of multiple dominant follicles [5, 6].