Colorectal cancer is the second major cause of cancer-related death in the United States. The long time involved in progression of mucosal dysplasia from a small polyp to an invasive cancer and the ability to image the colon mucosa are features that make early detection and prevention of colorectal cancer by colonoscopy possible. Although colonoscopy has contributed to a marked decline in the number of colorectal cancer-related deaths, the protective effect of colonoscopy, when used in routine clinical practice, has not lived up to the expectations raised by carefully controlled prospective research studies. Therefore new systems that assess quality of colonoscopy are needed.