Microbial interspecies electron transfer (IET) refers to the electron exchange between electron-donating microorganisms and electron-accepting microorganisms that forms a syntrophic growth relationship between the two thus enabling the two to jointly accomplish a certain metabolic process that no single microorganism can do. Moreover, it also plays a significant role in biogeochemical processes, such as degradation of organic matter, production of bioenergy and reduction of greenhouse gas emission. IET could be sorted into direct IET (DIET) and indirect or mediated IET (MIET). DIET occurs when there is a biological electrical connection and a difference in voltage potential, whereas MIET relies on diffusion of redox carriers driven by concentration gradients. Generally MIET needs hydrogen, formate or flavin as electron carrier, while DIET is found done directly through nanowire (e-pili), redox protein or conductive particles. Interspecies hydrogen/formate transfer, one type of MIET, occurs commonly in methanogenic microbial community, such as S organism and Methanobacterium ruminantium, Desulfovibrio vulgaris and Methanosarcina barkeri. In addition, sulfide, L-cysteine and AQDS can act as electron shuttles mediating electron transfer between microorganisms, such as Desulfuromonas acatoxidans and Prosthecochloris aestuarii. However, electron transfer between Geobacter species so far has only been documented to be direct: by way of e-pili and c-type cytochromes. Either of these Geobacter cells short of biological connections, such as e-pili and (or) cytochromes, can not get syntrophically related. Nevertheless, with the mediation of conductive materials, such as activated carbon and biochar, e-pili would become less functional during the process of DIET since syntrophic partners could exchange electrons via these conductive carbon materials. Moreover, conductive mineral magnetite can substitute for outer-membrane c-type cytochrome in its role. Mutant strain of G. sulfurreducens that is deficient in OmcS cannot co-culture with G. metallireducens, but with the addition of magnetites they can exchange electrons successfully. The discovery of DIET has changed the tradition gnosia that microbial syntrophic metabolism would not occur without energy carriers, such as hydrogen and formate, and has opened up a new scientific perspective for understanding biogeochemical processes, such as circulation of C/N/S, emissions of greenhouse and degradation of pollutants. The core of microbial IET is electron transfer between microbes. Further studies should be done on mechanism of IET and new effective IET microorganisms in order to put IET into practical engineering application. However, the researches on tmechanism of IET between microbes, at present, are still in their preliminary stage and so have a number of problems to be solved, for example, how exactly electron transfer occurs between microorganisms, whether there is any microorganism more IET efficient, and if there is any method that can more economically and efficiently accelerate IET, etc. In this review, the mechanisms of MIET is summarized, meanwhile, the three mediating mechanisms for DIET are expounded emphatically. Representative microbes participating in IET are introduced. Potential applications of IET to environment processes such as methane-producing anaerobic digestion, anaerobic methane oxidation and dechlorination are proposed and directions of future researches on IET discussed.