Cryopreservation of mammalian animal embryos was first developed using mouse embryos in 1972 and has since been applied to human embryos to utilize surplus embryos in in-vitro fertilization programs: the slow freezing method in the first half of the 1980's and vitrification in the 1990's. Recently, the protocol of vitrification for human embryos has been improved by the ultra-rapid vitrification method in which the rate of cooling is highly improved. It is mainly used as an effective basic technique for cryopreservation of human embryos in current clinical IVF programs due to the extremely high embryonic survival rate after storage.