Colonoscopy is a test that allows your doctor to look at the inner lining of your large intestine (rectum and colon). A colonoscopy helps find ulcers, colon polyps, tumors, and areas of inflammation or bleeding. During a colonoscopy, tissue samples can be collected (biopsy) and abnormal growths can be taken out. Colonoscopy is also used as a screening test to check for cancer or precancerous growths in the colon or rectum (polyps).