The cryopreservation of mammalian embryos has expanded over the past 20 yr by encompassing a range of sophisticated methods to deal with different developmental stages and different sensitivities to low-temperature exposure. We have described a method for slow, controlled-rate freezing of early stage embryos based on exposure to 1,2-propanediol and sucrose, while the method for late-stage (blastocyst) embryos employs mixtures of glycerol and sucrose. Both methods have been used for animal and human embryos. A third rapid cooling or "vitrification" technique is described, which depends on brief but controlled exposure of multicellular embryos to mixtures of glycerol and 1,2-propanediol at high concentrations. This technique is used for successful animal embryo cryopreservation but is not yet widely applied in the clinic.