The technique of embryo transfer is now routinely employed in several species and with several objectives. In laboratory animals the technique is used mainly in investigations of reproductive biology, whereas in human beings it is used to overcome specific forms of impaired fertility. Because embryo transfer combined with superovulation of the donor can significantly increase the female reproductive rate its greatest application to date has been in farm animals, in which it is widely used in both research and commercial production. Within the past few years there have been many advances in the techniques used in farm animals, particularly in the area of embryo manipulation. The supply of embryos can now be increased by repeated superovulation and by embryo bisection and there has been significant progress in in vitro fertilisation technology. Deep freeze (-196 degrees C) storage of embryos is now routine and may be combined with direct one-step thawing and removal of cryoprotectant. This technique allows the routine non-surgical transfer of embryos in the field. There is also evidence that a routine non-traumatic procedure for sexing embryos may soon be developed. Identical multiplets or clones have been produced by the microdissection of embryos and more recently by the transfer of nuclei from embryos into unfertilized oocytes. Transgenic animals have been produced by the microinjection of recombinant DNA into one of the pronuclei of single-cell unfertilized ova.