Embryo cryopreservation stands for the freezing of an embryo suspended in cryoprotectant and the storage of the frozen embryo in a cryotank. Often more suitable embryos than the one or two selected for replacement are cultured in an IVF cycle. Good quality embryos not replaced can be frozen, and thawed in a subsequent cycle to give more chances for a pregnancy. In the past most embryos were frozen using a programmable apparatus to freeze the embryos, in what is called the slow freezing method. Today most embryos and blastocysts are frozen by vitrification a rapid freezing method that results in lower cell damage, therefore embryos with greater pregnancy potential.