The toxicity of most halogenated solvents is associated with their biotransformation. The bioactivation can be due to the formation of stable, toxic metabolites or the formation of reactive, electrophilic intermediates. The paper summarizes present knowledge on the biotransformation of several halogenated compounds, ie, methyl halides, methylene halides, haloforms, carbon tetrachloride, chlorinated ethanes, and chlorinated alkenes. Some of the relevant enzymatic metabolic reactions are also discussed.