This chapter discusses the potential role of the new group of medications called aromatase inhibitors in assisted reproduction. When an aromatase inhibitor is applied during controlled ovarian hyperstimulation (COO), estrogen production per growing ovarian follicle has been found to be significantly lower than when aromatase inhibitors are not used. The use of aromatase inhibitors for in vitro maturation is an exciting application that can involve a brief aromatase inhibitor-induced rise in endogenous gonadotropin secretion leading to multiple ovarian follicles, followed by retrieval of immature oocytes. Both lowering supraphysiological levels of estrogen during COH and improving response to COH by enhancing endogenous gonadotropin production and increasing the ovarian follicular sensitivity to gonadotropin stimulation could be of benefit in particular groups of patients, for example, poor responders, endometriosis-associated infertility, polycystic ovarian syndrome (PCOS), and survivors of estrogen-dependant malignancies, for example, breast cancer.