Radiobiology embraces a diversity of interests. To understand its nature, we must contend not only with the subtlety and uncertainty of biology but also with the physicochemical events that are relevant to the absorption of concentrated packages of energy. While radiobiology is, implicitly if not explicitly, concerned with the action of all radiations on living systems, in practice, the high energy radiations command the greatest attention. Actually, all of the radiations are closely related , for to be effective each must function as activation energy whether this results from the highly specific molecular excitation with ultraviolet or from the more nonspecific transfer of energy by ionization and excitation with x-rays. In this report, our interest will be confined to the high energy or ionizing radiations and we will be concerned, for the most part, only with contribu tions that have appeared during the past year. Radiation effects on genetic materials and on micro-organisms are treated elsewhere in this volume ,and will not be considered in detail here. Complementing and highlighting the individual investigations during 1950 were several symposia devoted to radiobiology. The basic aspects of radiation effects on living systems was the subject of a symposium held at Oberlin College under the auspices of the National Research Council. Several sessions of the 6th International Congress of Radiology, the 50th Anni versary Celebration of the Discovery of Radium, and the annual meeting of the American Association for the Advancement of Science were likewise devoted to radiobiological matters.