Phenol, or hydroxybenzene, is both a synthetically and naturally produced aromatic compound. Microorganisms capable of degrading phenol are common and include both aerobes and anaerobes. Many aerobic phenol-degrading microorganisms have been isolated and the pathways for the aerobic degradation of phenol are now firmly established. The first steps include oxygenation of phenol by phenol hydroxylase enzymes to form catechol, followed by ring cleavage adjacent to or in between the two hydroxyl groups of catechol. Phenol hydroxylases ranging from simple flavoprotein monooxygenases to multicomponent hydroxylases, as well as the genes coding for these enzymes, have been described for a number of aerobic phenol-degrading microorganisms. Phenol can also be degraded in the absence of oxygen. Our knowledge of this process is less advanced than that of the aerobic process, and only a few anaerobic phenol-degrading bacteria have been isolated to date. Convincing evidence from both pure culture studies with the denitrifying organismThauera aromaticaK172 and with twoClostridiumspecies, as well as from mixed culture studies, indicates that the first step in anaerobic phenol degradation is carboxylation in thepara-position to form 4-hydroxybenzoate. Followingpara-carboxylation, thioesterification of 4-hydroxybenzoate to co-enzyme A allows subsequent ring reduction, hydration, and fission. Para-carboxylation appears to be involved in the anaerobic degradation of a number of aromatic compounds. Numerous practical applications exist for microbial phenol degradation. These include the exploitation of indigenous anaerobic phenol-degrading bacteria in the in situ bioremediation of creosote-contaminated subsurface environments, and the use of phenol as a co-substrate for indigenous aerobic phenol-degrading bacteria to enhance in situ biodegradation of chlorinated solvents.