Enzymes participating in oxidation and reduction of environmental pollutants such as phenols, azo dyes, nitroaromatics and aromatic amines are reviewed. Monooxygenases, enzymes, incorporating one atom of dioxygen into the molecule of substrate, consist of two groups. One of them includes enzymes containing flavins as prosthetic groups, while members of the other group are mixed-function monooxygenases containing heme enzyme cytochrome P450 as a terminal oxidase. Both types of enzymes play a major role in oxidation of xenobiotics in animals and microorganisms, while an additional group of heme enzymes, peroxidases, play a minor role. Dioxygenases utilizing both atoms of a dioxygen molecule are efficient enzymes biodegrading many xenobiotics, mainly in microorganisms. Reductases such as NADPH:cytochrome P450 oxidoreductase, xanthine oxidase and NAD(P)H:quinone oxidoreductase participate in biotransformation of azo dyes and nitroaromatics in reductive reactions. NADH, NADPH, xanthine or hypoxanthine are employed as cofactors in these reactions. Detailed study of structure and function of the reviewed enzymes might be utilized in modulating biodegradation of xenobiotics in organisms.