A colonoscopy is a medical procedure that doctors use to look at the inside of a person's large intestine (colon). An endoscope is a long, thin, flexible tube with a light and a camera on the end of it. The camera sends pictures to a computer screen so the doctor can see the inside of the colon. During a colonoscopy, the doctor may also take pictures or samples of tissue (biopsy) to be tested in a laboratory.