Chromium is of great environmental concern because hexavalent chromium is a potent carcinogen. The health effects resulting from chromium exposure have received as much attention as those resulting from benzene or asbestos exposure. Although considered innocuous, Cr(III) can be mutagenic and genotoxic when inside the nuclei of cells in target organs. The paper discusses the various oxidation states of the metal in the environment, the interconversion of Cr(III) and CR(VI), the sources of chromium in the environment, and the transport of the metal in the media. Chromium's tendency to change oxidation states, in both environmental media and living cells, is emphasized. It is possible for Cr(VI)to become immobilized in aquifers in a solid phase as barium chromate. This may present a problem in remediation by 'Pump and Treat' technology if the solid phase cannot be easily solubilized. The microbial reduction of hexavalent chromium and bioremediation of chromium-contaminated soil, sediments, and aquifers is summarized.