The basis for any rational scheme of x-ray therapy is a knowledge of the dose at every point throughout the treated tissue. The purpose of this paper is to review the data and methods available to make such dosage assessments. In a problem of such complexity, it is helpful to consider first a simple case, the case in which the irradiated tissue is homogeneous and, effectively, infinitely large. To anticipate, we will conclude that under these restrictions the available data and methods are adequate for clinical purposes. There is general agreement that for radiotherapeutic purposes such a homogeneous block of tissue may be approximated by a tank or phantom of water. It would thus be possible — but most laborious — for each radiotherapist to calibrate every port and every setting of his own equipment by direct measurement in a water tank. Fortunately it has been found that, in so far as the distribution of dosage in the beam is concerned, an x-ray beam can be characterized for clinical purposes by the target-skin distance (T.S.D.), the half-value layer of the radiation (h.v.1), and the port. This seems to be especially true of modern machines having beams which are symmetrical about the beam axis and with a sharp drop-off of radiation at the edges, in contrast to the wide and variable penumbras and the anode-cathode asymmetries found on older sets. There is a copious amount of data in the literature on the dose along the central axis of a square or round portal as a function of depth under the surface, tabulated over a wide range of half-value layer, target-skin distance, and port area. Unfortunately, the agreement among tables of different experimenters is not striking—illustrating, incidentally, how difficult it would be for each radiotherapist to construct his own. The two most widely used tables are those of Mayneord and Lamerton (1) and the tables given by Quimby (2) and supplemented by Braestrup et al. (3) (called in this paper the Quimby-Braestrup tables). The differences between these tables are large enough to be clinically significant, the Mayneord and Lamerton tables frequently giving values 10 or 15 per cent of the depth dose higher than the other set. On the basis of the measurements of Oliver and Kemp (4), it appears quite certain that the Quimby-Braestrup tables are the more nearly correct, and it would seem unlikely that the values given there will have to be changed substantially in the light of future measurements. The most customary and convenient way of giving dosage in the radiation field off the central axis seems to be in the form of so-called “isodose charts,” of which an example is shown in Figure 1. For a circular field only a single such chart plotting the dose in a plane containing the central axis of the beam is necessary. For rectangular fields two charts are usually employed to give the dosage distribution in the two planes running through the central axis and parallel to the long and short sides of the port.