The purpose of this communication is to present, in convenient form, values for the half-value depths and fall-off ratios corresponding to the entire range of radiation qualities, portal areas, and target-skin distances likely to be encountered in clinical roentgen therapy. Compilation of these data is believed to be useful for several reasons. First, these parameters have proved extremely useful in specifying superficial radiation beams, and it is thought that a graphic comparison of the data will facilitate treatment planning for superficial lesions. Second, though these values have seldom been considered in the planning of intermediate or deep therapy, they do provide, in some measure at least, an index of the distribution of radiation of higher energy in tissue. It is believed, therefore, that the data will enable the student to visualize readily the effect of changing kilovoltage, external filter, portal size, and target-skin distance on the distribution of radiation within a patient's tissues. Third...