Selenium is an essential component of glutathione peroxidase and other enzyme systems and is therefore essential for animal metabolism. It is, however, toxic at concentrations little above those required for health. Some plants accumulate selenium from seleniferous soils and constitute a toxic hazard to grazing stock. The more complex organic selenium compounds that occur naturally in plants, animals and microorganisms have been characterized following their isolation or partial isolation, and have often been identified by their similarity to analogous sulphur compounds. The occurrence and identification of selenium compounds in biological samples and methods for their analysis are reviewed.