Microbial fermentations allow transforming waste into compounds of interest such as biofuels or platform molecules and are thus essential building blocks for future environmental biorefineries. These fermentations sometimes seem to be very sensitive to an extracellular electron transfer from an electrode or from electroactive microbes, a phenomenon known as electro-fermentation (EF). In these cases, the additional electron sinks or sources modify the fermentation balance of the fermentative metabolism and provide new options for the control of microbial activity. EF has thus been used for increasing yields in various products such as hydrogen, acetate, propionate, butyrate, lactate, 3-hydroxypropanoic acid, ethanol, 1.3-propanediol, 2,3-butandiol, butanol, or acetone in either pure or mixed culture fermentations. In this talk I will present and discuss our recent results on EF using either electrodes or electroactive microorganisms. In particular, we unveiled that the EF effect in cocultures of Clostridium pasteurianum and Geobacter sulfurreducens is not only due to an electron exchange but also to a cobamide molecule produced by G. sulfurreducens, which probably interferes with the key enzyme glycerol dehydratase. I will also introduce a theoretical framework for the analysis and control of fermentations using EF and show how, in some experimental configurations, a dynamic control is required for the maximization of the desired product.