Metallothionein (MT) is a small, cysteine-rich, metal-binding protein. MT synthesis is induced by various stimuli such as heavy metals, oxidative stress, anticancer drugs and fasting stress. MT is capable of not only reducing metal toxicity but also scavenging free radicals. In fact, MT is involved in the protection of tissues against various forms of oxidative injury, including radiation, lipid peroxidation, oxidative stress caused by anticancer drugs, and conditions of hyperoxia. However, MT still lacks a manifest established biological function. Recently, transgenic mice with loss-of-function mutations in the MT-I/-II genes were established. Unexpectedly, the mice were in apparent good health, and the critical biological roles for MT have been questioned. Here the basic characteristics of MT are reviewed, the current MT study highlights summarized, and the putative biological functions of MT discussed.