Estrogens are essential regulators of female fertility. Estrogen biosynthesis is dependent upon expression of the cytochrome P450 aromatase, the highest levels of expression of which are detected in the human placenta and the granulosa cells of the mature ovarian follicle. Consistent with this pattern of expression, in non-pregnant premenopausal women, the ovaries have been shown to be the primary source of estradiol. In recent years there has also been an increased interest in the role(s) played by estrogens in regulation of adipose tissue, the vasculature and bone and as paracrine regulators of tissue function in postmenopausal women. In the premenopausal years the uterus is a target for estrogen and the cyclical event of menstruation is a consequence of the sequential exposure of the endometrium to estrogen and progesterone (reviewed in references 5 and 6). Endometrial breakdown occurs as a consequence of progesterone withdrawal with the demise of the corpus luteum (CL) in the absence of pregnancy.