Over the last two decades significant advances have been made in our understanding of the anaerobic biodegradability of monoaromatic hydrocarbons. It is now known that compounds such as benzene, toluene, ethylbenzene, and all three xylene isomers can be biodegraded in the absence of oxygen by a broad diversity of organisms. These compounds have been shown to serve as carbon and energy sources for bacteria growing phototrophically, or respiratorily with nitrate, manganese, ferric iron, sulfate, or carbon dioxide as the sole electron acceptor. In addition, it has also been recently shown that complete degradation of monoaromatic hydrocarbons can also be coupled to the respiration of oxyanions of chlorine such as perchlorate or chlorate, or to the reduction of the quinone moieties of humic substances. Many pure cultures of hydrocarbon-degrading anaerobes now exist and some novel biochemical and genetic pathways have been identified. In general, a fumarate addition reaction is used as the initial activation step of the catabolic process of the corresponding monoaromatic hydrocarbon compounds. However, other reactions may alternatively be involved depending on the electron acceptor utilized or the compound being degraded. In the case of toluene, fumarate addition to the methyl group mediated by benzylsuccinate synthase appears to be the universal mechanism of activation and is now known to be utilized by anoxygenic phototrophs, nitrate-reducing, Fe(III)-reducing, sulfate-reducing, and methanogenic cultures. Many of these biochemical pathways produce unique extracellular intermediates that can be utilized as biomarkers for the monitoring of hydrocarbon degradation in anaerobic natural environments.