The role of external-beam radiation therapy (EBRT) in differentiated thyroid cancer is reviewed. In the presence of gross residual disease after attempted surgical excision, retrospective series have reported local control is possible with EBRT. If, in addition to Iodine-131 (I131), there is a role for adjuvant EBRT in differentiated thyroid cancer, it would be only in patients in whom there is a high risk of relapse in the thyroid bed. Evidence is presented that suggests that EBRT can improve the local relapse-free rate in selected patients (over the age of 45, with microscopic residual disease or extensive extrathyroid invasion). For patients with recurrence in the thyroid bed, EBRT can be given in addition to surgery and I131. In bone metastases that are demonstrable radiographically, I131 therapy is often unsuccessful and EBRT also should be given. The technique of thyroid bed radiation is described. EBRT has acceptable acute toxicity and rarely produces serious long-term complications.