Ulcerative colitis is associated with an increased risk for colorectal cancer. The increase is approximately six to ten times the expected in the general population. Disease duration and extension are the major risk factors. Concomitant primary sclerosing cholangitis is an additional independent risk factor. The relative risk is approximately 14.8 for patients with pancolitis while it is 1.7 for patients with disease restricted to the rectum. The risk increases rapidly after 20 years of disease evolution. Active treatment might decrease the risk. The increased cancer risk is a major problem for longterm management of patients with ulcerative colitis. It is the rationale for various surveillance programs and the search for dysplasia. Two major types of dysplasia must be distinguished in ulcerative colitis: sporadic adenomas and ulcerative colitis-associated dysplasia. Sporadic adenomas can be treated by simple polypectomy. The diagnosis of dypsplasia relies mainly on microscopic analysis of biopsy samples. Additional techniques, including flow cytometry and the search for DNA abnormalities and immunohistochemistry or molecular techniques looking for genetic defects can help to improve the diagnostic yield.