In the context of the development of bioprocesses for the production of biofuels and bulk chemicals, microbial cells are rationally engineered to produce such molecules at high yield and titres in order to develop new biological methods that satisfy economic constraints. The redox and energetic balances of such strains play crucial roles in performance. Processes performed in strict anaerobes have a limited amount of energy available compared to that in aerobic organisms. This energy is obtained through fermentation and/or ion gradient-driven phosphorylation. Such anaerobic organisms have developed energy conservation mechanisms to increase ATP yields. This paper presents the properties of one of these mechanisms catalysed by the Rnf complex, an ion-translocating membrane complex with a ferredoxin NAD+ oxidoreductase activity. The Rnf complex performs the transfer of electrons from reduced ferredoxin to NAD+ coupled with an ion-motive transport. Ferredoxin is a common electron carrier for anaerobic bacteria and, with NAD+, is involved in several pathways of interest for the production of biofuels. This complex was first identified in Rhodobacter capsulatus and found to be involved in nitrogen fixation. It was then found to be involved in energy conservation in multiple anaerobic organisms, from acetobacteria such as Acetobacterium woodii to sulfate-reducing bacteria such as Desulfovibrio alaskensis and autotrophic bacteria such as Clostridium ljungdahlii and Clostridium aceticum. The Rnf complex triggers two types of ion transports: it can be either a sodium or a proton transporter. Both of these transports create a gradient of ions, generating a membrane potential that is then used by ATPase to produce ATP and thus serving as an energy conservation mechanism. In this review, the available information on the Rnf complex from genetic organization up to its in vivo and in vitro activities in several microorganisms is summarized, with a special focus on the proton-motive Rnf complex.