Radiotherapy has been integrated into the multi-modality treatment for rectal cancer. Postoperative radiotherapy provides the benefit in local control for Dukes' Bs, C1, and C2 diseases, but does not have survival benefit. Preoperative radiotherapy is associated with the advantages of reducing local recurrence rate, improving the resectability, and increasing the chance of anal preservation. Combined chemotherapy and radiotherapy further increases the local control rate. The comparison between preoperative and postoperative radiotherapy reveals the superior local control and anal preservation rates with preoperative radiotherapy. Most side effects from radiotherapy for rectal cancer are manageable and reversible.