The biological action of radiation undoubtedly constitutes an issue of actual con cern, particularly after incidences like those in Harrisburg or Chernobyl. These considerations, however, were not the reason for writing this book although it is hoped that it will also be helpful in this respect. The interaction of radiation with biological systems is such an interesting research objective that to my mind no special justification is needed to pursue these problems. The combination of physics, chemistry and biology presents on one hand a fascinating challenge to the student, on the other, it may lead to insights which are not possible if the dif ferent subjects remain clearly separated. Special problems of radiation biology have quite often led to new approaches in physics (or vice versa), a recent example is "microdosimetry" (chapter 4). Biological radiation a9tion comprises all levels of biological organization. It starts with the absorption in essential atoms and molecules and ends with the development of cancer and genetic hazards to future generations. The structure of the book reflects this. Beginning with physical and chemical fundamentals, it then turns to a description of chemical and subcellular systems. Cellular effects form a large part since they are the basis for understanding all further responses. Reactions of the whole organism, concentrating on mammals and especially humans, are subsequently treated. The book concludes with a short discussion of problems in radiation protection and the application of radiation in medical therapy. These last points are necessarily short and somewhat superficial.