Catechol dioxygenases are key enzymes in the metabolism of aromatic rings by soil bacteria. Catechol dioxygenases have been found that participate in the metabolism of halogenated aromatic compounds and, in doing so, play a key role in bioremediation of halogenated pollutants. The catechol dioxygenases can be divided into two major groups: those that cleave the aromatic ring between the vicinal diols (the intradiol enzymes) and those that cleave the ring to one side of the vicinal diols (the extradiol enzymes). Whereas both types of catechol dioxygenase contain an active-site iron that is required absolutely for enzymic activity, the intradiol enzymes contain Fe(III), while the extradiol enzymes contain Fe(II). The nature of the protein ligands determines this specificity. The differences in oxidation state of the active-site iron appear to result in mechanistic differences that lead to the differing regioselectivity of the two groups of catechol dioxygenase. Mechanistic proposals based on available evidence suggest a substrate-activation mechanism for the intradiol enzymes and an oxygen-activation mechanism for the extradiol enzymes.