The concept of relative biological effectiveness (RBE) and the physical and biological considerations in the determination of RBE are discussed, with particular reference to the RBE of high-energy photons and electrons. Earlier reviews are critically considered and recent measurements of RBE in this energy region are tabulated and discussed. It is concluded that there are no differences of practical significance between highenergy photons and electrons from 1 to 70 Mvp and that for most biological endpoints the RBE (with 200-kvp X-rays as the standard) is about 0.85 to 0.90.