This book contains the proceedings of a conference held at the Strahlenzentrum der Justus Liebig-Univer­ sitat, GieSen, Germany, from October 6 to October 10, 1975. About 40 sCientists, mostly from European coun­ tries, came together to discuss specific aspects of modern radiation biology. The majority of the papers deal with "simple eukaryotic systems" such as yeast, algae, slime moulds etc. It was felt that these study objects offer good prospects to tackle fundamental questions. This scope, however, was never intended to be exclusive, and the reader will see from the table of contents that a variety of different systems was discussed. Therefore we thought the general title would be appropriate. The rationale that led to the planning of the symposium and the titles of the ses­ sions is outlined in the Introduction. We hope that this volume gathers some useful information, not only for radiation biologists, but also for colleagues en­ gaged either in fundamental cell research or in the application in the field of radiation therapy and pro­ tection. The conference would not have been possible without the generous support of the European Communities and the cosponsorship of the Deutsche Gesellschaft fUr Biophysik. If it was to a certain extent successful, then the merit goes to the very active engagement of all participants. I should like to thank my coworkers for their devoted help in the organisation, particularly Dr. Eckart Schneider, Dr. Elmar Gocke, Maria Hlawica, and Eva-Maria Peter for secretarial help.