Many exciting opportunities have been created by the development of techniques associated with embryo transfer, for example, storage for prolonged periods by freezing, the production of identical offspring, and “chimaeras”. The possibility of obtaining increasing numbers of offspring from selected farm animals has attracted many breeders, and the possibility of being able to specify the sex of the offspring has excited attention even further. Similarly, the research worker welcomes the chance to manipulate reproduction and so investigate subjects as varied as factors which influence fertility, maternal effects on fetal growth and birth weight, sexual differentiation, and susceptibility to tumor growth.