The integration of radiotherapy to the adjuvant treatment of rectal cancer was prompted by the predominance of locoregional failures after curative surgery. This characteristic in the pattern of failure is one of the main reasons adjuvant radiotherapy plays a greater role in rectal cancer than in colon cancer. It has been demonstrated that local failure rates after surgery alone for rectal cancer are strongly dependent on the degree of bowel wall invasion, lymph node involvement, and margins of resection. These same locoregional factors are also predictive of distant metastasis and survival. In addition, local failure is associated with devastating symptoms that severely affect the quality of life of patients. For these reasons, locoregional control remains a major issue in the treatment of rectal cancer. This article summarizes the evidence that has established chemoradiotherapy as part of the standard of care for rectal cancer and the techniques used for its delivery.