From the first electromicrobial experiment to a sophisticated microbial electrochemical process - it all takes place in a reactor. Whereas the reactor design and materials used strongly influence the obtained results, there are no common platforms for MES reactors. This is a critical convention gap, as cross-comparison and benchmarking among MES as well as MES vs. conventional biotechnological processes is needed. Only knowledge driven engineering of MES reactors will pave the way to application and commercialization. In this chapter we first assess the requirements on reactors to be used for bioelectrochemical systems as well as potential losses caused by the reactor design. Subsequently, we compile the main types and designs of reactors used for MES so far, starting from simple H-cells to stirred tank reactors. We conclude with a discussion on the weaknesses and strengths of the existing types of reactors for bioelectrochemical systems that are scored on design criteria and draw conclusions for the future engineering of MES reactors. [GRAPHICS] .