A colonoscopy is an outpatient procedure where a long, thin, flexible tube or "scope" is inserted into the rectum and advanced through the entire colon (large intestine). The scope has a light and a camera on the end of it which allows your doctor to examine the lining of your colon. A colonoscopy may be performed to diagnose the cause of gastrointestinal symptoms such as abdominal pain, diarrhea, bleeding or abnormal x-ray results. A screening colonoscopy may also be performed on a patient at age 45 or sooner, depending on the patient's history, to screen for colon cancer and polyps.