Publisher Summary This chapter summarizes the previous evidence and presents the major advances of developmental toxic effects induced by excess iodine in animals and humans. Adequate iodine intake is indispensable to the production of thyroid hormone (TH), which is essential for growth, development and cell differentiation. Iodine excess, as well as iodine deficiency, has adverse effects on health. Fetal and maternal thyroid systems interact mainly by means of the placenta. When a mother is exposed to excess iodine, alterations in maternal–fetal TH metabolism occur. Animals can tolerate high doses of iodine exposure. Developmental toxic effects induced by excess iodine vary greatly in different species. No apparent or significant developmental toxic effect has been reported in humans when exposed to excess iodine. However, for pregnant women who consume high doses of iodine through food or drinking water or who are exposed to iodine-containing medication during pregnancy, the total iodine intake and their TH status should be closely monitored. Excess iodine exposure during pregnancy may influence maternal–fetal TH metabolism and result in fetal/neonatal hypothyroidism or hyperthyroidism; therefore, the thyroid function and morphology should be carefully evaluated in neonates when their mother is exposed to excess iodine during pregnancy.