Radiotherapy is an essential mode of treatment of many cancers, either alone or in conjunction with other methods such as surgery and chemotherapy. While in most cases radiotherapy is given using external radiation sources, it is also possible to administer radiotherapy by specifically localizing radioisotopes emitting particulate radiation within the tumour tissue. This mode of targeted therapy has several potential advantages over external beam therapy, including the possibility of delivering doses more selectively to the tumour and treating widespread multiple metastases. Such targeted therapy of tumours, as well as of other, benign conditions, was the thrust of nuclear medicine in the early years. A few therapeutic procedures for the treatment of hyperthyroidism and thyroid cancer introduced at that time continue to be used regularly. In the light of the major scientific achievements in this field in the past decade, targeted therapy using radiopharmaceuticals is now being actively pursued at many centres around the world for the management of cancer.