Microbial cometabolism, i.e. "transformation of a non-growth substrate in the obligate presence of a growth substrate or another transformable compound" (Dalton and Stirling 1982) is a whole-cell phenomenon physiologically based on coupling of different catabolic pathways at the cellular level. It is frequently observed in transformation of xenobiotic non-growth substrates by individual microbial species. Transformation processes of this type are usually mediated by appropriate non-specific enzymes of the peripheric cellular metabolism able to modify a variety of substances other than their natural substrates. The precise mechanisms of coupling between metabolism of xenobiotic non-growth substrates and of particular additional carbon substrates may be different depending on the substrates and the microbial species involved. However, experimental data indicate that the primary function of the respective additional carbon substrates is to supply either energy, cofactors or metabolites for the different cellular events involved in the transformation process (e.g. uptake of the xenobiotic non-growth substrate, functioning of appropriate degradative enzymes of the peripheric cellular metabolism). Cometabolism of xenobiotics involves nothing special or novel from the standpoint of biochemistry. On the contrary, there are numerous examples where the turnover of particular natural compounds by certain aerobic or anaerobic microorganisms is essentially based on coupling of different catabolic pathways at the cellular level by transfer of hydrogen (i.e. reducing power) and/or energy between two or more enzymatic reactions. Synthetic chemicals which resist total degradation by individual microbial species may undergo mineralization due to complementary catabolic sequences mediated by certain multispecies microbial associations with cometabolic transformations being the initial steps. Although taking place in certain natural habitats (e.g. rhizospheres, sewage), microbial cometabolism of xenobiotics in natural ecosystems occurs with slow rates since the respective cometabolizing populations are generally small and will not increase in number or biomass in response to the introduced chemicals. However, under conditions of axenic microbial cultures, high concentrations of biomass, and appropriate substrate mixtures cometabolism of synthetic chemicals may be a useful technique of considerable practical importance to accumulate biochemical products at high yields. In addition, cometabolic capabilities of wild-type microorganisms may serve as a tool for the construction of microbial strains with a new degradative potential for recalcitrant xenobiotic compounds.