This article describes an investigation in radiography in the range from 0.5 to 2.5 million volts. The work described began in the summer of 1941 as a contribution to national defense by the Massachusetts Institute of Technology and was later continued and extended under contract with the National Defense Research Committee to meet an urgent radiographic need in the war program, particularly for the Navy. Beginning in 1937, the High Voltage Laboratory at Massachusetts Institute of Technology produced and investigated X-rays of over two-million volts (2 Mev) in connection with nuclear research. Thus, the project could be quickly converted into the exploration of the radiographic possibilities of these penetrating X-rays and into the development of techniques by which they could be applied to the war effort. The expected advantages of the higher voltages were soon confirmed; and, in addition, surprising new advantages were discovered. These results led the National Defense Research Committee to contract with M.I.T. for the development and production of five, compact, 2-Mev X-ray generators. The contract also provided for other work necessarily associated, including the development of new practical radiographic techniques, the servicing of the equipment, and the training of personnel, so that the new generators and their radiation could be used in the field by the Navy with maximum effectiveness. The special war contribution of this radiographic project lay in the fact that, for the first time, it was possible for the Navy to examine the interior of large numbers of the various types of its heavy explosive weapons, including mines, torpedoes, shells, bombs, and rockets. This nondestructive method of examination of these vital weapons made it possible to improve steadily their construction and performance as they were produced and sent out for use against the enemy. This newly introduced range of radiography was also extensively applied to the examination of the interior of many explosive heavy weapons of a particularly hazardous nature captured from the enemy. Moreover, it was possible to examine quickly and with great precision the interior design of optical and other scientific equipment.