Because toxicity varies enormously with the chemical state of metals, transformations in the environment control the level of the human health hazard. Important transformation processes include adsorption and desorption from soils and sediments, oxidation and reduction (redox) reactions, biotic metabolism, formation of organic metal compounds, and bioaccumulation. The six metals detailed in this chapter--arsenic, cadmium, chromium, lead, mercury, and selenium--were chosen because of their toxicity, frequency of occurrence at hazardous waste sites, and involvement in environmental contamination.