Radiation therapy combines the aspects of an art, whereby each patient is considered as an individual problem, with various personal characteristics which must be taken into consideration, and of a science, in which a great deal of definite information is available. Clinical medicine alone can offer training in the first; radiological physics contributes largely to the second. Since an x-ray generator or a radium applicator is a physical apparatus, it is evident that it can do its best work only for those who understand its physical possibilities. The American Board of Radiology, recognizing the importance to the radiotherapist of an acquaintance with fundamental physics of radiology, has required its candidates to pass an examination on this subject. It has, therefore, been necessary that residents and fellows in radiology receive instruction along these lines. There are no satisfactory textbooks, and few trained teachers, in this field. Much of the advanced training of prospective radiologists is more or less along apprentice or tutorial lines, and many of these students must learn their physics in a rather haphazard fashion, from short courses at meetings, from books and papers, and from informal discussions. Great credit is due the many men and women who, in spite of such handicaps, have satisfactorily passed the physics examination. It is generally conceded that, largely as a result of the activities of the Board of Radiology, doctors are increasingly better trained before becoming specialists in this field. In no part of their subject is this more evident than in radiological physics. Examinations have been conducted in this branch for seven years; during that time, failures have steadily decreased—and not because of increasing leniency on the part of the examiners. The average passing candidate at present has a definitely greater fund of knowledge and the ability to put this to practical use. In this connection, acknowledgment should be made of the value of the so-called refresher courses offered by the American Roentgen Ray Society and the Radiological Society of North America. Many men whose access to teaching institutions is limited acquire in these courses basic information which enables them to carry on alone to much better advantage. There has come to be less disconnected memorizing of definitions and isolated facts and more general logical effort to understand the field as a whole. Year after year, however, as candidates come before the physics examiners for the Board, a definite percentage of them invariably present certain errors and misconceptions. Some of these are due, of course, to carelessness on the part of the student, but some are due to neglect or misplaced emphasis on the part of teachers or in the literature. It seems that an analysis of some of these stock difficulties, and some suggestions about overcoming them, should be useful to both instructors and students.