Although a method exists for freezing sheep embryos directly into liquid nitrogen without a programmable freezing machine (1 ), a more traditional slow cooling method is described here. Either dimethyl sulfoxide (DMSO) or ethylene glycol can be used as a cryoprotectant in this slow cooling method (2 ). Pregnancy rates for frozen embryos are similar to those of fresh embryos. For a general discussion of cryopreservation, the reader is referred to Chapter 25.