Nitroaromatic compounds are released into the biosphere almost exclusively from anthropogenic sources. Some compounds are produced by incomplete combustion of fossil fuels; others are used as synthetic intermediates, dyes, pesticides, and explosives. Recent research revealed a number of microbial systems capable of transforming or biodegrading nitroaromatic compounds. Anaerobic bacteria can reduce the nitro group via nitroso and hydroxylamino intermediates to the corresponding amines. Isolates of Desulfovibrio spp. can use nitroaromatic compounds as their source of nitrogen. They can also reduce 2,4,6-trinitrotoluene to 2,4,6-triaminotoluene. Several strains of Clostridium can catalyze a similar reduction and also seem to be able to degrade the molecule to small aliphatic acids. Anaerobic systems have been demonstrated to destroy munitions and pesticides in soil. Fungi can extensively degrade or mineralize a variety of nitroaromatic compounds. For example, Phanerochaete chrysosporium mineralizes 2,4-dinitrotoluene and 2,4,6-trinitrotoluene and shows promise as the basis for bioremediation strategies. The anaerobic bacteria and the fungi mentioned above mostly transform nitroaromatic compounds via fortuitous reactions. In contrast, a number of nitroaromatic compounds can serve as growth substrates for aerobic bacteria. Removal or productive metabolism of nitro groups can be accomplished by four different strategies. (a) Some bacteria can reduce the aromatic ring of dinitro and trinitro compounds by the addition of a hydride ion to form a hydride-Meisenheimer complex, which subsequently rearomatizes with the elimination of nitrite. (b) Monooxygenase enzymes can add a single oxygen atom and eliminate the nitro group from nitrophenols. (c) Dioxygenase enzymes can insert two hydroxyl groups into the aromatic ring and precipitate the spontaneous elimination of the nitro group from a variety of nitroaromatic compounds. (d) Reduction of the nitro group to the corresponding hydroxylamine is the initial reaction in the productive metabolism of nitrobenzene, 4-nitrotoluene, and 4-nitrobenzoate. The hydroxylamines undergo enzyme-catalyzed rearrangements to hydroxylated compounds that are substrates for ring-fission reactions. Potential applications of the above reactions include not only the biodegradation of environmental contaminants, but also biocatalysis and synthesis of valuable organic molecules.