Well differentiated thyroid cancers (DTC), usually having an indolent course, are generally treated by surgery, i.e., total or near total thyroidectomy, followed by radioiodine and TSH suppressive therapy with thyroid hormone. The beneficial effect of external beam radiotherapy (EBRT) in the treatment of selected metastatic sites (i.e., brain and bone) or for palliation in cases of locally advanced inoperable disease is widely accepted. In contrast, its efficacy in improving postoperative locoregional disease control is still controversial. A better definition of subgroups of patients at high risk of local failure is mandatory. At present, patients older than 40-45 years affected by papillary cancers with macro- or microscopic postoperative residual disease and with extensive extrathyroid invasion appear to benefit from EBRT performed in addition to surgery and radioiodine. The role of EBRT in patients with radioiodine non-responsive progressive disease will also be discussed.