In the decade following the publication of the first edition of Cellular Biology of the Uterus, advances in this field have been so rapid as to require not merely a revision of the earlier text but an essentially new volume. Even the title of the book has been changed, to Biology of the Uterus, to reflect the incorporation of more material based on classical anatomy and physiology. This histological and embryological information provides a necessary, though often lacking, background for the protein chemist and molecular biologist, and a bridge between biochemistry and biophysics, on the one hand, and clinical medicine, on the other. Thus, major practical problems in human reproduction, such as the mode of action of contraceptive agents and the cause of the initiation of labor, may be approached on a firm scientific footing. This text deals primarily with the biology of the uterus itself (comparative and human) rather than with placentation or pregnancy, and as such is a synthesis of data derived from many techniques, conventional and modern. Inasmuch as it is clearly beyond the competence of anyone scientist to prepare such a text on the basis of personal knowledge and experience, the aid of distinguished biologists from this country and abroad was enlisted. All of these authors, acknowledged experts in their respective fields, agreed to extensive revision of their chapters or preparation of entirely new contributions.