Since the Ciba Foundation Symposium in 1952 on Mammalian Germ Cells when Dr Audrey Smith reported that 'exposure to very low temperatures is not incompatible with further development of mammalian eggs', much progress has been made on the problem of freezing mammalian embryos. The significant steps leading to this progress are reviewed and an attempt is made to assess the extent of our current knowledge and to relate this to experience gained in other fields of low temperature biology. There is good evidence that certain basic principles concerning cooling and warming rates are applicable to the preservation of all mammalian embryos so far studied, but differences between species and between stages of development within species exist, particularly in their resistance to cooling in temperature ranges above 0 degrees C. Some of these differences are illustrated by reference to experiments with pig embryos. Clearly there are many problems remaining to be solved, but practical applications of techniques for long-term storage of mammalian embryos are already feasible and offer interesting possibilities for future development.