A colonoscopy is a medical procedure where a doctor looks at the inside of your large intestine (colon) and rectum to check for problems. They use a thin, flexible tube called an endoscope, which has a light and a camera on the end of it. The doctor will look for things such as polyps (growths), ulcers, and bleeding. If they find anything abnormal, they may do a biopsy (take a small sample of tissue) to test for cancer. In some cases, they may also remove the growth (polyp) using the endoscope.