The receptive phase of the endometrium seems to occur in close association with the appearance of pinopodes and endometrial integrins that may be activated by the interleukin-1 system (IL-1). Embryo attachment is the result of adhesion protein expression, and the invasion of the embryo is governed by proteolytic enzymes. Leukaemia inhibitory factor (LIF) is produced by natural killer lymphocytes that interact with the invading trophoblast. This may activate urokinase plasminogen activator (uPA) and gelatinase enzymes, which play a crucial role in trophoblast invasion. Oestrogen stimulates, while progesterone inhibits, LIF. The role of endometrial contractility in displacing human embryos from the Fallopian tube to the lumen cavity of the uterus or vagina in terms of pregnancy or wastage is still a matter of discussion. Endometriosis and its associated abnormal uterine contractions may be also linked to implantation failure. Unless new evidence emerges to indicate otherwise, it can be assumed that progesterone is, either in a direct (non-genomic: contractility) or indirect (genomic: decidualization) manner, the only determinant of endometrial priming necessary for embryo nidation.