Colonoscopy is a major advance in the diagnosis, treatment, and investigation of diseases of the colon and rectum, particularly cancer. Based on our early experience, this technique can be expected to bring malignant and premalignant lesions of the large intestine to clinical attention at an earlier stage and thus effect a higher cure rate. The large majority of colonic polyps can be removed from all levels of the colon safely by the endoscopic route, eliminating the need for laparotomy in most cases and reducing costs of care materially. Errors in radiologic diagnosis, positive, negative, or equivocal, can he materially reduced when there is endoscopic confirmation. Thus, colonoscopy and the contrast enema may be regarded as complementary studies. Colonoscopy is an extremely safe method of study when carried out in expert hands: it can be done on an ambulatory basis. There is an urgent need for good training programs to ensure adequate availability of the method to the public. The technique is also an excellent means of followup for patients who have been treated for cancer, polyps, or inflammatory diseases of the colon and rectum. Pertinent and illustrative case material gleaned from a clinical experience of over 4000 colonoscopies and 600 endoscopic polyp removals is presented.