Embryo cryopreservation is now firmly established as a routine component of in vitro fertilization (IVF) and other assisted conception techniques for the resolution of human infertility. Excess fertilized oocytes and embryos can be preserved for infertile couples avoiding the necessity to replace large numbers of embryos which reduces the incidence of multiple births and their sequelae. The additional pregnancies obtained from replacement of cryopreserved embryos has effectively increased the success rate of a cycle of IVF treatment for a couple by 1 to 10%. There is an increasing preference to cryopreserve fertilized pronuclear oocytes by slow cooling in 1,2-propanediol (PROH) although many clinics also cryopreserve early cleavage stage embryos by slow cooling in PROH or dimethyl sulphoxide (DMSO) with similar success rates to those for pronuclear oocytes. Numerous problems have appeared with the attempted cryopreservation of mature unfertilized oocytes, including nonreversible disassembly of the meiotic spindle during cooling, reduced fertilization rates due to the effects of cryoprotectants and cooling on the zona pellucida, parthenogenetic activation of oocytes by PROH, substantially increased polyploidy and aneuploidy when frozen-thawed oocytes are fertilized and increased cryoinjury to unfertilized oocytes when compared to fertilized oocytes. It is also necessary to address problems which have arisen through the disagreement of separated couples about the disposition of embryos in cryostorage.