Buried sediments are generally characterized by the absence of oxygen and so microbial processes will be dominated by the activity of anaerobic bacteria. Carbon turn over in the presence of electron acceptors therefore involves interaction of the carbon, nitrogen and sulphur cycles. In the absence of electron acceptors other methods of energy conservation become significant. Interspecies H 2 -transfer permits the use of otherwise energetically unfavourable reactions. Conservation of the energy in polyphosphate bonds and reduction of Fe (III) and Mn (IV ) to produce more energetically favourable end-products are other mechanisms available to benthic bacteria. The reduction of CO 2 to acetate may require more serious consideration as a hydrogen sink in certain sediments. The list of substrates known to be susceptible to attack by anaerobic bacteria has grown rapidly in recent years and estimates are now available for the turnover of refractory components such as lignin. Finally, bacteria are considered as producers of biomass, particularly of specific cell components that may be used as biomarkers to identify zones of activity. The key features of such biomarkers are identified.