Heat stress is the major cause of low fertility of cattle worldwide; it is a multifactorial problem. Pronounced alterations of ovarian functions are recorded in heat-stressed cattle. Among others, they include attenuation of follicular dominance, depression of steroidogenesis, low oocyte quality, and low progesterone production that is reflected in low concentration in plasma. New evidence indicates that heat stress has a long-term, delayed effect on ovarian follicles that alters their function several weeks after termination of the stress. The above alterations may impair embryo development directly or indirectly, and lower fertility during the summer and autumn.