Colonoscopy is a minimally invasive endoscopic procedure that lets your doctor examine the lining of your large intestine (colon) for abnormalities. During colonoscopy, your doctor inserts a thin, flexible tube into your rectum and slowly moves it through the entire length of the colon. This instrument, called a colonoscope, has a camera and light source and allows your doctor to look at the lining of your colon and rectum.