The purpose of this book is to describe the nature of the materno-fetal immunobiological relationship and to suggest the direction in which the management of reproduction and its failure in man is moving. The several authors, who have written about their special fields of interest, need to be read within a framework designed to blend their contributions into a whole. This preamble provides a part of that framework, by describing the early development of the embryo, that of the placenta and its membranes and their anatomical relationship with maternal tissues: in other words, the stage upon which this materno-fetal dialogue takes place. Professor Maureen Young's 'tour de force', encapsulating the whole of fetal physiology into a single chapter, completes the background information. After Maureen Young's summary of fetal physiology, Matteo Adinolfi describes the development of the immune system in the fetus, including new information that allows more accurate speculation concerning the gestational age at which fetal immune responses of various kinds may begin. Charles Loke examines the nature of antigens which are found in the placenta, concentrating on those which occur on syncytiotrophoblast and suggesting roles for them in fetal development. Arnold Klopper covers the wide range of proteins and hormones which have been studied during pregnancy and found to vary in a potentially significant way. He has been careful to distinguish between observation and hypothesis, as far as any immunomodulating action is concerned, and his analysis is a model of scientific scepticism.