The belief that energy might be a limiting factor for the development of humanity led twenty years ago to a great interest being'taken in research on anaerobic digestion. The first international symposium held in Cardiff in 1979 was followed by the meetings in Travenmund (1981), Boston (1983), Guangzhou (1985) and Bologna (1988). By now anaerobic digestion has come to be recognized as an appropriate technology for waste treatment. More recently, the increase in the carbon dioxide content of the atmosphere and (in developed countries, especially_ in the EEC) the· fact that more and more land is becoming available for purposes other than food production make biomass production economically and/or socially feasible for industrial purposes. The possibility of using renewable organic carbon resources in this way is of great potential interest for developing biological techniques and could considerably increase the use of anaerobic micro-organisms in cellulose biotransformation and energy production from crop residues. This FEMS Symposium is devoted to the interspecies hydrogen transfer phenomenon involved in the mineralization of organic matter in anaerobiosis. This process is carried out in Nature by consortia of anaerobic micro-organisms living syntrophically. Many industrial applications of these consortia as black boxes for biogas production and waste treatment have been described. Although these early approaches were fruitful, it seems likely that a better knowledge at the molecular level of the more characteristic anaerobic bacteria which constitute these consortia would greatly increase and improve the utilization of these organisms.