The roentgen examination of the colon, including the rectum, if properly done and if conditions present are accurately demonstrated, interpreted, and evaluated, is one of the most difficult—probably the most difficult—of all radiologic examinations. Even so, it is the most widely used and accurate single method employed in definitive diagnosis of colonic disease. It demands skill, experience, wisdom, and judgment in interpretation, painstaking roentgenoscopic observation during filling, detailed care in the radiographic phase of the examination, and more than the average amount of hard work. That the examination be thorough, and so planned and executed that any abnormality demonstrable by the roentgen ray will be detected, is of utmost importance, for the large bowel may be the site of many diseases. Not a small percentage of these can be eradicated or effectively controlled if discovered and accurately diagnosed, especially when recognized in a relatively early stage. Many bowel disturbances are entirely functional, but even in these the roentgen examination is essential for ruling out organic causes for the patient's complaints. Cancer is foremost in the minds not only of the medical profession but of the laity as well. Approximately 17 per cent of all deaths caused by cancer are traceable to the colon and rectum, and of these approximately 75 per cent are the result of disease of the rectum, the rectosigmoid, and the sigmoid. Polyps have approximately the same percentage incidence as cancer in the various parts of the large bowel, and are always potential sites of malignant growth. The importance of their early detection and adequate therapy cannot be too strongly emphasized. Diverticulosis of the colon is a relatively common finding in mid and late adult life. In itself it may not be serious except when infection and inflammation of one or more of the diverticula supervene, giving rise to an acute diverticulitis which, in turn, may be followed by a pericolonic abscess and at times perforation into the peritoneal cavity. The pericolonic inflammatory reaction causes narrowing of the adjacent lumen of the bowel and may result in a high degree of partial or even a complete obstruction. The deformity seen in acute diverticulitis may closely simulate that due to carcinoma, and from the radiological standpoint the differential diagnosis, in a few instances, may be most difficult. Diverticulosis and cancer are infrequently encountered in the same individual, and concurrent diverticulitis and cancer are rare indeed. This is fortunate not only for the patient but for the examiner as well, for an accurate differential diagnosis may be impossible in the latter instance. No attempt will be made to consider, even briefly, the many lesions with primary manifestations in the colon, for they are generally recognized.