Microbial fuel cells (MFCs) provide new opportunitiesfor the sustainable production of energy from biode-gradable, reduced compounds. MFCs function on differ-ent carbohydrates but also on complex substratespresent in wastewaters. As yet there is limited infor-mation available about the energy metabolism andnature of the bacteria using the anode as electronacceptor; few electron transfer mechanisms have beenestablished unequivocally. To optimize and developenergy production by MFCs fully this knowledge isessential. Depending on the operational parameters ofthe MFC, different metabolic pathways are used by thebacteria. This determines the selection and performanceof speciﬁc organisms. Here we discuss how bacteria usean anode as an electron acceptor and to what extentthey generate electrical output. The MFC technology is evaluated relative to current alternatives for energy generation.