Colonoscopy for bleeding indications (positive fecal occult blood test, emergent or nonemergent rectal bleeding, melena with a negative upper endoscopy and iron deficiency anemia) has a substantial yield for cancers (1 per 9 to 13 colonoscopies), although slide rehydration of fecal occult blood tests decreases the yield (1 per 45 colonoscopies). Prospective studies indicate that nonbleeding colonic symptoms have a substantially lower yield for cancer than bleeding symptoms (1 per 109 colonoscopies). Patients with indications for screening colonoscopy with a relatively high yield of cancer are those with Lynch syndrome (1 per 39 colonoscopies) and males more than 60 yr old (1 per 64 colonoscopies). Perioperative colonoscopy in persons undergoing colorectal cancer resection has a high yield for synchronous cancer (2-3%). An initial examination in persons with long-standing ulcerative colitis has a high yield for cancer (12%). Surveillance colonoscopy after cancer resection has an intermediate yield for anastomotic cancer (1 per 74 procedures) and metachronous cancers (1 per 82 colonoscopies), although this number may overestimate the yield of metachronous cancer. Postpolypectomy surveillance and ulcerative colitis surveillance colonoscopy have relatively low yields for cancer (1 per 317 and 360 colonoscopies, respectively). However, postpolypectomy surveillance colonoscopy, in combination with initial clearing colonoscopy, has been proven to be almost entirely effective in preventing colorectal cancer death. Further, cancer yields for postpolypectomy surveillance should improve with implementation of new surveillance guidelines, with little or no impact on mortality. The effectiveness of ulcerative colitis surveillance is less certain. Referral of patients with low-grade dysplasia for colectomy would improve the value and effectiveness of surveillance colonoscopy in ulcerative colitis. Adenoma yields at colonoscopy are relatively independent of indication, as evidenced by the high yield of adenomas in screening colonoscopy studies. Demographic factors, including increasing age and male gender, are important predictors of adenomas at initial colonoscopy. Age, male gender, and multiple and large adenomas at initial examination are predictors of adenomas at subsequent postpolypectomy surveillance. Persons with ureterosigmoidostomies can be screened by interval flexible sigmoidoscopy and do not require screening colonoscopy. A history of breast cancer does not predict an improved yield of colonoscopic screening compared to average-risk persons.