The performance of a microbial fuel cell (MFC) depends on a complex system of parameters. Apart from technical variables like the anode or fuel cell design, it is mainly the paths and mechanisms of the bioelectrochemical energy conversion that decisively determine the MFC power and energy output. Here, the electron transfer from the microbial cell to the fuel cell anode, as a process that links microbiology and electrochemistry, represents a key factor that defines the theoretical limits of the energy conversion. The determination of the energy efficiency of the electron transfer reactions, based on the biological standard potentials of the involved redox species in combination with the known paths (and stoichiometry) of the underlying microbial metabolism, is an important instrument for this discussion. Against the sometimes confusing classifications of MFCs in literature it is demonstrated that the anodic electron transfer is always based on one and the same background: the exploitation of the necessity of every living cell to dispose the electrons liberated during oxidative substrate degradation.