Cometabolism recognizes that microorganisms can transform non-growth-supporting substrates. The term “cometabolism” was first introduced over 30 years ago and has been redefined, criticized, and used widely ever since. In this review we have examined the aerobic cometabolism of chlorinated solvents, with a particular emphasis on the cometabolism of trichloroethylene. Monooxygenases or dioxygenases with relaxed substrate ranges initiate these transformations. The physiological role of the oxygenases is to initiate the metabolism of growth-supporting substrates (e.g., methane, propane, butane, toluene, ethylene, and ammonia). Diverse enzymes catalyze these oxidative reactions with chlorinated solvents. Synthesis of most of these enzymes is induced by the presence of the growth-supporting substrate and is largely regulated at the level of gene transcription. The genes that code for a given oxygenase are usually clustered together in a single operon and often share homology with counterparts that code for the subunits of related oxygenases in other bacteria. During cometabolism the growth-supporting and non-growth-supporting substrates can both bind to the oxygenase. Transformation of chlorinated solvents by these enzymes presents the cell with a new set of compounds. Some of these compounds are toxic to the cells, others are stable products that are expelled from the cell, and in a few cases the cells utilize the products. The combined effects of cometabolism can have a profound influence on a cell.