Polycyclic aromatic hydrocarbons (PAHs) are among the most important contaminants of groundwater. The 2- and 3-ring PAHs are of particular concern because they are water soluble in the 1–200 μg/l range and are transported with the groundwater over significant distances. Anaerobic degradation of PAH has been demonstrated in several microcosm studies with nitrate, ferric iron, or sulfate as electron acceptors and under methanogenic conditions. The biochemical degradation pathways were studied with naphthalene-degrading pure and enrichment cultures and revealed that 2-naphthoic acid is a central metabolite. Naphthalene is activated by addition of a C1-unit to generate 2-naphthoic acid, whereas methylnaphthalene is activated by addition of fumarate to the methyl group and further degraded to 2-naphthoic acid. In the central 2-naphthoic acid degradation pathway the ring system is reduced prior to ring cleavage generating e.g. 5,6,7,8-tetrahydro-2-naphthoic acid. The ring cleavage produces metabolites such as 2-carboxycyclohexylacetic acid indicating that further degradation goes via cyclohexane derivatives and not via aromatic compounds. Anaerobic degradation of PAH has also been demonstrated in situ in contaminated aquifers by identification of compound specific metabolites and using stable isotope fraction studies. Detection of specific metabolites of anaerobic PAH degradation such as naphthyl-2-methylsuccinate indicated anaerobic degradation of 2-methylnaphthalene in situ whereas 2-naphthoic acid was indicative of naphthalene and 2-methylnaphthalene degradation. Other carboxylic acids that were detected in groundwater indicated anaerobic degradation of a wide range of PAH and heterocyclic compounds. Degradation of naphthalenes in contaminated aquifers could also be confirmed by carbon stable isotope shifts in the residual substrate fraction.