Spare embryos that remain available after an embryo transfer during ovarian stimulation/IVF cycles may be cryopreserved at extremely low temperatures (-196º) and remain intact for a long period of time. The cryopreservation procedure may be applied in 1 day-old embryos (at the pronucleate stage) or later in 2 or 3 day-old embryos. Whenever the couple decides so, an embryo thawing program may be arranged with an 80% of cryostored embryos usually surviving, and subsequently a thawed embryo transfer with good chances to achieve pregnancy can be scheduled. Thaw cycles are organized in a way to synchronize the development stage of thawed embryos with the endometrial receptivity. A thaw cycle can be totally ‘natural’, with a simple monitoring of the developing follicle during a woman’s natural cycle and the estimation of the time when her endometrium becomes receptive to the embryo in order to proceed to embryo transfer; or it can be ‘artificial’, where the woman’s natural cycle is inhibited and the necessary estrogens and progesterone are administered by us in order to enable the endometrial receptivity and perform the embryo transfer. It should be noted that the use of frozen-thawed embryos has not been associated to any risks of disorders in children born from those embryos.