The question of deciding what is the best dose to use in X-ray therapy is as unsettled as some of the international problems with which we are faced to-day, but, by reasonable consideration, accurate observation and recording of reactions and results, and the free co-operation without resort to violence which should be possible amongst radiotherapists, it should not be difficult to settle the radiological problems, even if words like insuperable are used of the international ones. As an introduction to the question, I propose to give some idea of the working hypothesis which has been guiding my footsteps in radiotherapy for the past six years, built up on information from so many sources that I cannot acknowledge them in detail in this short note. Any fundamental research, published or unpublished, chance observations in clinical papers, and my own clinical experience based on personal knowledge of all the patients treated in a clinic which has dealt with about 9000 patients since 1931, have all contributed to this hypothesis. The action of radiation must be physico-chemical, since its effects are produced by the electro-magnetic radiation and the secondary electrons only, which change the energy of electrons or displace electrons completely from the atoms of which they are part. This is another way of expressing the fact that radiation causes the ionisation by which it is measured and that the impact of a beam of radiation gives rise to secondary or recoil electrons.