Numerous elements, especially metals, possess biological activity. But until now, they were of interest mainly as ingredients of foods. This caused the grouping in essential and nonessential elements, and characteristic changes of their concentrations in tissues and body fluids of man are often accompanied by pathologic alterations. The growing interest in environmental influences on human, animal, and plant health has focused our view on the toxicity of certain elements. Often it depends on concentration (1) or chemical form (e.g., Cr3+ is essential whereas Cr6+ is venomous) if an element is vital or poisonous.