Aromatic compounds in the environment can be of natural or anthropological origins. Xenobiotic arenes are found to be weakly degraded because of the presence of stable aromatic ring (due to the delocalization of their p orbitals) and different constituents which can impede biodegradation rate. That's why the cleavage of aromatic ring by dioxygenases of bacterial origin is the critical step in removing of theses xenobiotics from environment. Also, monooxygenases play important role in biotransformation of the initial structure to one of the central intermediates: catechol, hydroquinone, protocatechuate or gentisate. In biodegradation of haloaromatics, dehalogenases are the essential enzymes in removing these xenobiotics.