By virtue of its high metabolic heat production (4–5 times maintenance), the lactating dairy cow is very sensitive to heat stress. A rise in body temperature of 1–2°C can have negative effects on various aspects of reproductive function. Heat stress can reduce luteinizing hormone and steroid hormone secretion, alter folliculogenesis, disrupt oocyte quality and inhibit embryonic development. The net result is reduced expression of estrus and low fertility. Effects on oocyte quality take place over a broad time frame. The ovarian follicle requires about 4 mo to develop from the primary stage to ovulation. The effects of heat stress on the oocyte within the follicle can persist for as long as 103 d. This stress in turn alters the integrity of the follicle and consequently makes it more difficult for the fertilized oocyte to develop into a blastocyst. The newly formed embryo is also susceptible to maternal heat stress. After 2–3 d, however, the embryo becomes increasingly resistant to heat stress and by d 7, when embryo transfer is ordinarily performed, elevated temperature has little effect on the embryo. Still another effect of heat stress is the depression of estrus behavior where both the duration and the intensity of estrus decrease resulting in an increased percentage of missed estruses. The combined effects of reduced detection of estrus and reduced fertility after insemination result in lower conception rates during warm months. These negative effects of heat stress can be bypassed by incorporation of 2 reproductive technologies. Ovulation synchronization protocols such as Ovsynch allow farmers to predict the time of ovulation without the need for estrus detection. Fertility is improved when synchronization is performed in conjunction with embryo transfer. Embryo transfer bypasses causes of infertility due to loss of oocyte competence and damage of the early embryo due to elevated maternal body temperature. The d 7 embryo used for transfer has acquired resistance to heat stress; therefore, maternal heat stress no longer affects the embryo.