Electro-fermentation (EF) is a novel process that consists of electrochemically controlling microbial fermentative metabolism with electrodes. The electrodes can act as either electron sinks or sources that allow unbalanced fermentation. They can also modify the medium by changing the redox balance. Such electrochemical control exerts significant effects not only on microbial metabolism and cellular regulation but also on interspecies interactions and the selection of bacterial populations in mixed microbial cultures. In this paper we propose some basics and principles to better define the EF concept within the field of bioelectrochemistry. We also explore the up-to-date strategies to put EF into practice and propose hypothetical mechanisms that could explain the first EF results reported in the literature.