The first studies on metallothionein were carried out during the 1950s and 1960s. In the past 10 years, there has been an increasing interest in metallothionein among scientists in the field of toxicology, environmental hygiene, medicine, pathology, biochemistry, and nutrition. A vast number of publications dealing with different aspects of metallothionein have appeared, far too many to cover within the scope of this chapter. The purpose of this presentation is to provide certain basic information about metallothionein which may be of value for a better understanding of the mechanisms involved in cadmium metabolism and toxicity.