Since 1966, at roughly three-yearly intervals, an international group of immunologists has met somewhere in Europe to discuss the latest developments in our understanding of the mechanisms governing the functioning of the immune system in vivo. These meetings have become known as the International Conferences on Lymphatic Tissues and Germinal Centers in Immune Reactions, or for the regular devotees, simply as the Germinal Center Conferences (GCC). This volume represents the proceedings of the 8th GCC, which was held in Babraham, near Cambridge, UK, between the 14th and 17th August, 1984.When one considers how cellular immunology has become increasingly dominated by in vitro methodology over the past twenty years, it may seem remarkable that these conferences have survived at all, let alone prospered. However, I for one do not find this surprising, since I suspect that the exquisitely complex architecture and microenvironments of the lymphoid system will never be fully understood through in vitro studies. If "the proper study of mankind is man", then surely ultimately the proper study of all the interacting elements which comprise the immune system has to be in vivo.This belief is shared by a substantial number of immunologists, as the contents of this volume will attest. Although the GCC were originally devoted to unravelling the mysteries of the germinal center response (and these are still not fully resolved), over the years the scope of the meetings has inevitably broadened. Thus, the present conference covered not only the development of B lymphocytes, but also T cell differentiation, the structural basis of immune induction and immunoregulation by products of lymphoid cells. Finally, to add a slightly applied finishing touch to the proceedings, there were sessions devoted to human lymphocyte subpopulations, and to the potentials of monoclonal antibodies as therapeutic agents. I hope that a perusal of this volume will convince you that in vivo cellular immunology is still alive and well, and that the Germinal Center Conferences continue to provide a unique forum for work in this important field.