To produce a therapeutic-biological action by means of X rays, a certain amount of physical energy must be absorbed in the radiated strata. In skin diseases this reaction occurs in the affected layers. But physiological experiment as well as clinical experience have shown that in many general conditions, disturbances of the more deep-seated organs can be influenced by way of the surface. Great interest has been displayed during the last twelve years in that part of the long-wave X-ray therapy which is known as Grenz or Bucky rays. It is proposed to discuss the literature and also the writer's own experience, and the results of his experimental work since 1930. In many University hospitals on the Continent and in America, superficial X-ray therapy is carried out mainly, if not entirely, by means of Grenz rays. Grenz-ray therapy has many advantages when compared with treatment by X rays. In the first place, the degree of healing is usually much more satisfactory. The technique is easier. The apparatus is not costly to purchase, as is the case with X rays, and it can be maintained with but little expense. With Grenz-ray therapy there is a very low incidence of residual damage, and lastly, there is no difficulty in repeating treatment in the case of recurrence of the disease. Grenz-ray treatment may be applied locally or generally. The local treatment is applied directly to the affected part of the organism. This is the case especially in skin diseases.