The current understanding of abiotic and biotic chemistry of halogenated aliphatic compounds is systematized and summarized. Knowledge of abiotic transformations can provide a conceptual framework for understanding biologically mediated transformations. Most abiotic transformations are slow, but they can still be significant within the time scales commonly associated with ground-water movement. In contrast, biotic transformations typically proceed much faster, provided that there are sufficient substrate and nutrients and a microbial population that can mediate such transformations. Recent studies, which describe transformations of halogenated aliphatic compounds in microbial and mammalian systems, are also discussed.