With the 20-million-volt electron beam of good intensity now produced in the University of Illinois betatron, questions about the practical use of high-energy radiations can be examined. The most promising way to use the betatron in therapy would be to send the original electrons accelerated in the vacuum tube directly into the patient. At 20 million volts these electrons will penetrate as far as 10 cm. and no farther. Thus no damage is done to the back of the patient. Furthermore, the ionization should reach a maximum 7 or 8 cm. beyond the entrance surface for the electrons, and the damage could be well localized within the body. About a 25- or 30-million-volt betatron would be ideal for this work, since it has the right energy and a reasonable size. Although a sufficiently intense beam of electrons now comes out of the betatron, it is not yet in a good enough state of collimation or control for practical use. The x-rays produced by this electron stream when it strikes a target cause an ionization intensity about as great as that used in practical therapy, while the distribution of ionization in thick sections of tissue-like material shows features very advantageous for deep therapy. The chief characteristics of the high-energy depth-dose curves are: (1) that the point of maximum dose is as deep as 3 or 4 centimeters below the surface of the phantom; (2) this maximum can be several times greater than the surface dose. X-rays in this energy range, therefore, will pass not only through the skin but also through the fat layer under the skin without producing much damage, while the tissue deeper in the body and near the point of maximum ionization will receive a large dose. The desired region in the patient can be placed near the point of maximum ionization intensity, if necessary by compression of his body, and cross-fire will be less important. In this discussion only experimental results with x-rays and their explanation are presented. Suggestions about dosage measurement, which must be well understood, are also given, since these high-energy radiations have properties which do not show up at lower energies. For an understanding of the behavior of x-rays of ordinary therapy voltages and also of betatron voltages it is necessary to consider only the Compton absorption process, since it is predominantly responsible for the interaction between these x-rays and tissue-like material. The absorption of an x-ray quantum will produce a secondary Compton electron and a secondary x-ray quantum in all cases. At low voltages, such as 400 kilovolts, the ionization appears very close to the point where the Compton process occurs, since the resulting electron has only about a 1-millimeter range. The shape of the depth-dose curve is not influenced by the electron range, since it is short. The number of x-ray quanta present at different depths therefore determines the shape of the depth-dose curves, and the low-voltage analysis is based on this fact.