The advent of radiation as a therapeutic agent in neoplastic diseases has stimulated an enormous amount of interest in the fundamental nature of the action of radiation on tissues in general and on tumors in particular. The actual intimate mechanism by which radiation produces injury to cells is vague, and its elucidation must await prolonged biophysical analysis. On the other hand, much information is available regarding the various types of cells or tissues which are especially susceptible to injury by radiation, together with certain of the conditions under which they exhibit this susceptibility. The histologic changes following irradiation have been abundantly described. Much of this knowledge has come from the study of the behavior of tumors under radiation, and from these observations on the behavior of tumors have been formulated certain generalizations which have been called laws of radiosensitivity. It must be realized that these laws are expressions of observed