Iodine deficiency has been found to result in mental retardation in children as well as an enlarged thyroid gland (goitre) and sometimes deficiencies in thyroid hormones in people of all ages. This review focuses on 26 studies with a comparison group of iodine supplementation in children. The quality of the studies was found to be generally poor. One study suggested a reduction in infant mortality. In some studies there was a trend towards better developmental outcomes after iodine prophylaxis. There was some concern in studies using iodised salt that small children may not eat enough salt to achieve adequate iodine status. Adverse effects were reported, although most of them were minor and did not last long. It was concluded that more high quality long term studies measuring outcomes related to child development, to deaths associated with iodine-deficiency, and to intervention programmes relevant to children in developed countries, are needed.