▪ Abstract The prokaryotes (bacteria) comprise the bulk of the biomass and chemical activity in sediments. They are well suited to their role as sediment chemists, as they are the right size and have the required metabolic versatility to oxidize the organic carbon in a variety of different ways. The characteristic vertical nutrient (electron donor and electron acceptor) profiles seen in sediments are produced as a result of microbial activities, with each nutrient a product or reactant of one or more metabolic groups. Thus, understanding the mechanisms by which the chemical environment of a sediment is generated and stabilized requires a knowledge of resident populations, something that has been very difficult to obtain, given the techniques available to microbiologists. However, the new approaches of molecular biology, which have added insights into the phylogenetic relationships of the prokaryotes, have also provided tools whereby sedimentary populations can be examined without the need for culturing the organisms. These techniques, in concert with new methods of microscopy, isolation of new metabolic groups, and the study of new ecosystems, suggest that there is much that will be learned about the microbiology of sedimentary environments in the coming years.