Adjuvant therapy of differentiated thyroid cancer with radioactive iodine ((131)I) is a standard procedure for the ablation of remnant thyroid tissue following surgery and for the treatment of iodine-avid metastases. Presently, there are two dosimetric concepts for the treatment of thyroid cancer using radioiodine: a) the bone marrow dose limited approach and b) lesion-based dosimetry. Both concepts and their clinical applications are described. In addition, the use of (124)I as a diagnostic and dosimetric agent is discussed. Treatment of children and adolescents with radioiodine requires special precautions; individualized approaches in this setting are reviewed. The limitations of treatments aiming at high absorbed doses are addressed as well as the doses to normal organs. Finally, new concepts for further elaborating the potential of thyroid cancer treatment using (131)I are introduced.