Hydrogen and formic acid have been considered as the intermediate electron transporters among microbes for a long time. In recent years, however, it has been found that direct interspecies electron transfer (DIET) might be an alternative beyond hydrogen/formic acid to transfer electron among microbes. As a new way of electron transfer among microbes, the electron transfer efficiency of DIET is higher than that of traditional hydrogen/formate transfer. The discovery of DIET has changed the traditional understanding that the growth and metabolism of microbial syntrophism must rely on electron carriers such as hydrogen or formic acid, and also has opened a new perspective for the study of microbial interaction. Although great progress has been made in the study of DIET, in-depth studies are still lacking on the microbes that can form co-culture via DIET, the mechanism of DIET, and the factors affecting DIET. In this review, we summarized the microbes that can form DIET, the mechanism underlying the extracellular electron transfer of microbe acted as electron donor in DIET, as well as the mechanism underlying the extracellular electron transfer of microbe acted as electron acceptor in DIET. The effects of conductive materials on DIET were elaborated, and several research directions for DIET were proposed, with the aim to mitigate performance degradation and facilitate research and development in this area.