Metallothioneins (MT) are low−molecualr weight (about6—7ku) and cysteine−rich (30%) metal−binding proteins, and their synthesis can be induced by radiation exposure. Biological functions of MT include mainly micronutrient, storage, transportation and metabolism, resistance to ionizing radiations, scavenger of free radicals and metal detoxification. The paper gives a review on MT classification, MT genes structure, gene amplification and regulation, radiation induced synthesis of MT and the mechanisms, and protective effects of MT to ionizing radiations.