The author's purpose is to explain, for students and physicians, the physical principles basic to an understanding of radiotherapy. The first chapter on the nature of matter and of radiation is followed by six chapters on roentgen rays, one on energy absorption, two on radium, one on high energy devices such as the betatron, and one on artificial radioactivity with emphasis on the therapeutic use of materials like cobalt 60 that emit gamma rays. Two appendixes supply a wealth of precise technical information in the form of tables and diagrams. There are many good illustrations, a subject index, and an author index, and the book as a whole is legible and attractive. It can be recommended both as an introduction for students and as a reference work for specialists using ionizing radiations therapeutically.