The biological monitoring of metals, when used properly, allows total exposure to a particular metal to be measured from various media. It takes into consideration inter- and intraindividual variations in uptake due to differences in metabolism and physical work load and can be used to identify individuals, or groups of individuals, with high exposure or at high risk. As many metals are retained for long periods, biological monitoring may not only provide information on recent exposure, but also on exposure which occurred a long time ago. Under optimal conditions, the concentration of a metal in biological media can be used to assess exposure, the concentration of the metal in the target or critical organ (ie, the organ where the adverse effects are first observed) and the risks for adverse effects. This paper gives an overview of several important aspects of biological monitoring but does not provide detailed information on particular metals.