Progressing degradation of the natural environment has caused an increased interest in the field of biological treatment of both water and soil polluted with different xenobiotics. Aromatic compounds are considered to be one of the most toxic and weakly degraded xenobiotics. The potential solution of the accelerated environment's pollution problem seems to be the bioremediation - the process using the biological organisms to return the environment altered by contaminants to its original condition. Dioxygenases isolated from the microorganisms can be responsible for hydroxylation of the aromatic ring or for its cleavage. Different types of the cleaving dioxygenases have been distinguished due to the kind of the substrate they preferentially can degrade. Cleavage of the aromatic ring facilitates further degradation of xenobiotics, and therefore the dioxygenases are the key enzymes in the biodegradation process of these xenobiotics.