Microbial metabolism is responsible for the removal of persistent organic pollutants including polychlorinated biphenyls (PCBs) from the environment. Anaerobic dehalogenation processes of highly and moderately chlorinated biphenyls generate moderately and low chlorinated congeners, which can be subject to aerobic degradation, performed by enzymes of the biphenyl (bph) upper and lower pathways. These enzymes and their substrate specificities are discussed in Section 2.1. Biphenyl 2,3-dioxgenases are generally considered key enzymes of this pathway, which determine substrate range, and extent of PCB-degradation, however, also the specificity of subsequent enzymes is crucial for productive metabolism. PCB metabolism is described in detail for some model organisms, and the genetic organization of gene clusters of model organisms is described in Section 2.2. So far, the genomes of two important PCB metabolizing model organisms, namely Burkholderia xenovorans LB400 and Rhodococcus jostii RHA1 have been sequenced with the rational to better understand their overall physiology and evolution as described in Section 2.3. This has also allowed a better evaluation into genome and proteome-wide defenses against PCB toxicity, which is summarized in Section 2.4. However, more and more studies also indicate that our knowledge on enzymes and genes involved in PCB metabolism is still rather fragmentary and on overview of the diversity of enzymes reported and mosaic routes is given on Section 2.5. Finally, strategies to optimize microorganisms for improved PCB degradation and bioremediation are discussed in Section 2.6.FONDECYTFONDECYT