The implantation window is defined as that period when the uterus is receptive for implantation of the free-lying blastocyst. This period of receptivity is short and results from the programmed sequence of the action of oestrogen and progesterone on the endometrium. Implantation itself is a process that commences with apposition, continues through attachment to trophoblast outgrowth and decidualization. For maximal effectiveness of assisted reproductive technologies in women, it is important to know the optimal time for embryo transfer which implies a need to predict the period of uterine receptivity. At present there are no good markers of, or for prediction of, uterine receptivity. In cycles where endogenous hormonal activity is suppressed or absent, the optimal time for embryo transfer can be easily defined and lies between luteal days +3 to +5, where luteal day +1 is the first day of exogenous progesterone treatment. In the human, it is suggested that blastocyst apposition begins about LH day +6 and is complete by LH +10. Human embryos survive in vitro manipulation well, and the stage of development at which they are placed in the uterus seems less critical than in other species, provided they are at an earlier stage of development than that of the endometrium.