In the last few years the study of germinal centers of the lymphoid tissue has progressed at an accelerated pace. Questions about their role and their significance in immune responses that could not be answered, mainly because of tedmicallimitations, are presently approached experimentally from many different directions. Hypotheses, some more than half a century old, receive renewed attention. At this time, members of the Biology Division, Oak Ridge National Laboratory, U.S.A., and of the Institute of Pathology, University of Bern, Switzerland, decided to bring together workers interested in the field. The Conference was held in Bern, June 22-24, 1966, and included fifty-seven contributions which were discussed at length. The range of interest extended from phylogenesis and anatomy to studies on cell proliferation, immunohishemistry, cancer research and radiobiology. The aim of this broad coverage was to combine all available information on the role of germinal centers in immune responses in a single package, instead of leaving it scattered around in reports dealing with divergent immunological problems. This attempt is reflected in the present book. A rather large space has been devoted to the lively discussions which followed the reports, the volume of most of which had been voluntarily limited. The discussions are not reported verbatim but care has been taken to insure neutrality and objectivity in the necessary adaptation of the shortened transcription. We have been very fortunate indeed, to have Drs.