Under anoxic condition, a microorganism can transfer electrons directly to another microorganism (i.e., direct interspecies electron transfer or DIET) to couple the metabolic capability of the two microorganisms for their syntrophic growth. DIET between bacteria and archaea is the new way for energy exchange between bacteria and archaea as well as the new mechanisms for regulating their metabolisms. Furthermore, DIET between bacteria and archaea is directly involved in methane formation and the anoxic methane oxidation coupled to sulfate reduction. Thus, DIET between bacteria and archaea plays a crucial role in global biogeochemical transformation and cycling of carbon and sulfur. Finally, all currently available data suggest that multi-heme c-type cytochromes may form continuously extracellular electron transfer pathways to mediate electron transfer between the cytoplasmic membranes of bacteria and archaea via the multi-step hopping mechanism.