The energy absorbed by a material traversed by any ionizing radiation may be investigated at three levels of complexity: 1. The dissipation of energy may be considered as continuous in space, the only relevant quantity being then the dose in ergs per gram. 2. The dissipation of energy may be considered as continuous along the tracks of the ionizing particles, varying with their charge and velocity. The relevant quantities are then the ergs per gram, as before, and the ergs per centimeter of track, or the distribution of this quantity when it is not constant for all tracks. 3. The dissipation of energy may be considered as a series of discrete eventsionizations and excitations-occurring in the vicinity of the tracks of the ionizing particles. A complete description of the situation at this level would require a knowledge of the nature and spatial distribution of the separate events. Experimental measurements of dose by ionization chamber or thermal dosimeters will indicate only the dose in ergs per gram. The individual particle energies in an irradiated medium may be determined, within certain limits, by means of scintillation counters, but the determination of the structure of the tracks is not at present accessible to experimental techniques except in the case of gaseous media, where cloud chamber photographs give an indication of the longitudinal spacing of the ions (1-3) and measurements on the saturation curve for a-particle ionization give a rather indirect clue to the effective diameter of the tracks (4, 5). It has usually been assumed (6, 7), for want of exact knowledge, that the structure of the tracks in liquid media would resemble the structure deduced for gases, reduced in proportion to the relative densities of gas and liquid; and the rate of energy loss along the tracks has, therefore, usually been quoted in terms of ion pairs per micron, or some such unit. There are, however, serious theoretical objections (8, 9) to the view that the track structure is similar in liquids and gases, but no satisfactory theoretical picture of the structure of the tracks in liquids has yet been given. At the present time it is, therefore, less speculative to focus attention on the rate of energy loss of the particles, for which a satisfactory theory and accurate experimental observations exist, i.e., to concentrate on the second level of complexity. Zirkle (10) has proposed the term "linear energy transfer" (LET) for the rate of energy loss of the particle, which is, of course, the same thing as "stopping power" but with the