Chromium is ubiquitous in the human environment. its nutritional and toxic properties are closely aligned to the ionic form. Thus whilst trivalent chromium [Cr (III)] is recognised as an essential micro-trace element with a key role in the modulation of blood sugars as a “glucose tolerance factor” and insulin promoter, hexavalent chromium [Cr(VI)] is highly toxic both as a profound allergen in industrial exposure and as a carcinogen. Carcinogenicity of chromium has been subject to extensive research, and studies in mammalian cell cultures have established beyond all reasonable doubt that Cr(VI) can evoke mutagenic, clastogenic and genotoxic change, DNA adducts, etc. in a battery of tests. Cr(VI) is mutagenic in the Salmonella typhimurium reverse mutation test whereas inconsistent results have been seen with Cr(III) compounds. Epidemiological and experimental animal studies show that Cr(VI) is absorbed from the diet and following inhalation more readily than the reduced state [Cr(III)], and penetrates cell membranes better. Cr(VI) is a site-specific carcinogen in humans with clinical evidence of lung and respiratory tract tumours, and possibly gastric cancer. Carcinogenicity does occur in animal studies, but as in subcutaneous sarcoma and inhalation/bronchial implantation studies, doses or exposures are high and, in many cases, vastly exceed expected human exposures or routes of exposure. In an attempt to gain positive observations, animals have been exposed to Cr(VI) by routes that bypass physiological defence mechanisms. In human and animal studies, the carcinogenicity of Cr(VI) closely relates to the rate of intracellular reduction of the ion to the minimally toxic Cr(III), and the residual tissue concentrations of the unreduced ion. Cr(III) is modestly mutagenic and capable of inducing chromosomal aberrations in vitro, but has not routinely shown carcinogenic changes in animals or humans other than where it was contaminated with Cr(VI).