Since 1966, at ro ghly three-yearly intervals, an internation- al 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 C, mferences on Lymphatic Tissues and Germinal Centers in Immune Reactions, or for the regular devotees, simply as the Germinal Cent' r 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 increas- ingly dominated by in vitro methodology over the past twenty years, it may seem remarkable that these conferences-hav survived at all, let alone prospered. However, I for one do not find this surprising, since I suspect that the exquisitely: omplex architect- ure and microenvironments of the lymphoid system will never be fully understood through in vitro studies. If "tae proper study of mankind is an", 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 immunol- ogists, 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.