Radioiodine therapy has been successfully applied for over 50 years in the management of differentiated thyroid carcinoma. Careful patient preparation and selection of the optimal dose of radioiodine to be administered are two factors of major importance in the course of management. Main indications for 131I therapy are the ablation of residual thyroid tissue after thyroidectomy, the treatment of locoregional recurrence and distant metastases which involve almost exclusively the lung and bones. A controversial aspect is that of patients with high serum TG levels but negative whole body 131I scintigraphy for whom there is no general agreement. Other controversial aspects involve ablation therapy as the selection of patients to be treated and the control of its efficacy. The cost and possible adverse side-effects of 131I therapy require a careful analysis of prognostic factors in each patient candidate for the treatment.