Casual reading of contemporary medical records bearing on the action of roentgen rays and radium often gives the impression that little is known of such action in spite of the fact that substantial or conclusive experimental data are available to indicate or to establish clearly the nature of it. In some cases, indeed, the experimental indications are absolute. In other cases, the experimental evidence may be inadequate, but the clinical data may be sufficient to suggest the probable effect of irradiation. It is true that many problems relating to radiotherapy are still the subject of controversy, and that the large number of experiments that have been made have yielded only partial answers to many questions. Nevertheless, the facts already brought to light are sufficiently numerous to provide an imposing, although admittedly incomplete, scientific background. Unfortunately, the evidence furnished by experiments on animals and clinical observation has never been analyzed and