Abstract Epidemiological studies have demonstrated that nickel, chromium, arsenic, cadmium, and beryllium compounds are human carcinogens. Most of these epidemiological studies were done retrospectively in humans occupationally exposed to compounds of these metals. There are only a few environmental studies in humans exposed to any of these metal compounds in which possible speciation of the metal compound is considered. Of the compounds of the five metals, those of nickel, chromium, cadmium, and beryllium have been confirmed as carcinogens in vivo, in experimental animals. The genotoxicity and mutagenic activity of metals is discussed briefly; however, the reader is referred to more detail in chapters on individual metals and to Chapter 9, “Selected Molecular Mechanisms of Metal Toxicity and Carcinogenicity.” Cell transformation in tissue culture and other mechanistic information is summarized for each metal ion.