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Chlorinated aliphatic compounds, notably the chlorinated solvents, are common contaminants in soil and groundwater at hazardous waste sites. While these compounds are often recalcitrant, under favorable conditions they can be transformed and degraded through microbially mediated processes. There is great interest in understanding the transformations that are observed at contaminated sites and in manipulating these systems to achieve remediation. An important class of transformations occurs in anaerobic environments. Many of the transformations are reductive, and many yield useful energy to specific anaerobic bacteria. They include reductive dechlorination, dehydrochlorination and dichloroelemination. Of these, reductive dechlorination is often a growth-supporting reaction, while the others may be abiological or catalyzed by biological molecules. The reactions may result in chlorinated products, but there are often reaction sequences leading to completely dechlorinated products. The behavior of carbon tetrachloride (CT), 1,1,2,2-tetrachloroethane (TeCA) and the chloroethenes, perchloroethylene (PCE) and trichloroethylene (TCE), illustrate the range of anaerobic transformations that are possible, as well as the limited transformation that often is seen in the environment. CT undergoes reductive and substitutive reactions that are catalyzed by biological molecules but do not support bacterial growth. The anaerobic degradation of TeCA, which is a major contaminant at a site near Tacoma, WA, USA, provides examples of each type of transformation, and the products formed are consistent with the chlorinated compounds that are found in groundwater extraction wells. A laboratory study, using anaerobic sludge that had been fed chlorinated compounds, a cell-free extract from the sludge, and killed controls, showed that TeCA was transformed to four products and that these were further transformed, suggesting that it might be possible to degrade TeCA to innocuous products. Reductive dechlorination of PCE and TCE has been studied in many laboratories. Studies with mixed anaerobic consortia and with several dehalogenating bacteria, including strain 195 (. Isolation of a bacterium that reductively dechlorinates tetrachloroethane to ethane. Science 276, 1568-1571) that transforms PCE to ethene, have demonstrated that reductive dechlorination supports growth of the novel bacteria that carry out the reactions. Hydrogen has been shown to be an electron donor for the bacterial dehalogenation reactions, and kinetic and thermodynamic considerations indicate that dehalogenators can compete in some, but not all, anaerobic environments, pointing to applications of in situ bioremediation and to its limitations. Selected field studies of anaerobic transformations help delineate the applications of this type of bioremediation.