Radiotherapy has an established role in the treatment of rectal cancer. In primary resectable cancer, numerous randomised trials have shown that particularly pre-operative, and to some extent also postoperative, radiotherapy substantially reduces the risk of local failure. This is seen also with total mesorectal excision. Secondary to the reduction in local failures, there is also a slight improvement in survival after pre-operative radiotherapy or postoperative radiochemotherapy. Using appropriate techniques, the morbidity of radiotherapy is low. In non-resectable cancer, radiotherapy may cause down-staging, allow surgery, and may cure some patients. Whether radiochemotherapy is more efficient has yet to be firmly established. The role of pre-operative radio(chemo)therapy to permit more sphincter-preserving procedures with adequate long-term function is not defined.