Bio-fuel cell, or microbial fuel cells (MFCs) can convert chemical energy of organic compounds to electric power, with their aerobic degradation to CO₂ and water. MFCs are analogous to polymer electrolyte fuel cell (PEFC) in terms of the reaction sequences: fuel oxidation, proton transfer and oxygen reduction, whereas MFCs can be also considered as a waste treatment reactor. In this review, in comparison with PEFC, the items explained are the mechanism of MFCs, application of MFCs to wastewater/organic waste (rice straw and excess sludge) disposal, and application of “exoelectrogens” to mediator-less MFCs. Feasibility of MFCs is discussed for designing reactor structures of MFCs as semi-continuous, mediator-less, and single-chambered form.