Metals differ from other toxic substances in that they are not destroyed in nature. Furthermore, they may be transported over great distances and they have a tendency toward accumulation in nature, as well as in the human body. It is important to recognize that they are natural substances. Historically, metal toxicology largely concerned acute effects. The knowledge and understanding of such effects must continue to be developed, but they are uncommon in present safety standards. There is, however, a growing interest in the chronic toxic effects of metals, which may be subclinical or not readily obvious. Short-term exposure may produce clinical effects very different from those produced by similar exposure in terms of total dose over a longer period of time. Chronic poisoning may occur after occupational and environmental exposure. A number of factors influence the toxicity of metals, e. g. their chemical form or speciation (elemental state, inorganic compounds, metal complexes and organometal- lic compounds), the interactions of toxic with essential metals, the age and immune status of an individual. A critical determinant of a metal’s toxicity is its biological half-time. The biological half-times of cadmium and lead are 20 to 30 years, whereas for some metals such as arsenic, cobalt and chromium they are only a few hours or days. The critical organs affected by metals vary. Their most important effects after chronic exposure involve the lungs, kidneys, central nervous system, and immune system. Chronic effects of metals also include reproductive toxicity and carcinogenicity. Nevertheless, metals are naturally present in the environment and a major problem is to determine which levels of exposure can be accepted with only a minor, well-defined risk for human health.