In the past decade or more, few radiobiological quantities have received as much attention as the biological effectiveness (or RBE) of high-energy photons and electrons relative to conventional 200 to 300 kvp x-rays. Many of these studies have had a clinical rather than a radiobiological objective, namely the specification of the RBE for clinical therapy. No doubt it has often been felt that the effectiveness of a radiation measured, say, in the growth of broad bean roots or even in the survival of mice might have little bearing on its effectiveness as a therapeutic agent. In radiotherapy, the location and type of the lesion to be treated, the volume irradiated, the integral dose, the distribution of dose, the sparing of particular normal tissues, the dose fractionation technic, and many other factors in addition to RBE, must be involved in the choice of one type of radiation in preference to another. Nevertheless, the RBE is an important factor in guiding the thinking of the radiotherapist and in determi...