While the number of identified substances produced by the ovary increases steadily, it remains remarkable that the sole use of exogenous estrogen (E2) and progesterone (P) can prime optimal endometrial receptivity in women whose ovaries have failed or are absent. Early work showed that a marked leeway existed in the acceptable duration of the E2-only phase of endometrial priming. Subsequently, a sequence of transformations are induced by exogenous progesterone that reproduces classical findings made in the menstrual cycle. Secretory changes in endometrial glands are best seen between the 4th and 6th day of progesterone administration (day 18-20 of an ideal cycle where progesterone exposure starts on day 15). Predecidual changes of the endometrial stroma are apparent starting on the 10th day of progesterone exposure (day 24). Contrary to earlier belief, even maximal alterations in the plasma E2 to progesterone ratio fails to alter the endometrial morphology of either glands or stroma. More recently it has been recognized that E2 and progesterone also affect uterine contractility. It has been postulated that excessively high levels of E2 may increase uterine contractility and adversely affect implantation rates in in-vitro fertilization (IVF). Exogenous progesterone has been shown to exert utero-relaxing effects and it has been hypothesised that progesterone supplementation before embryo transfer (ET) may improve receptivity in IVF.