The application of the concept of a "controlled environment" led to the identification of eight trace elements with a proven or postulated biological function during this decade. Rigorous reduction of metallic contamination from air, drinking water, and diet is necessary to reduce exposure to the element under investigation below requirement levels. An essential function for a trace element is suggested when deficiency is produced, and indepently confirmed, in two or more animal species, and also when the signs of deficiency are prevented or cured by supplements. As yet, a direct role for the "new trace elements" in the human organism has not been demonstrated.