Bioelectrochemical systems (BESs), such as microbial fuel cells (MFCs) and microbial electrolysis cells (MECs), are generally regarded as a promising future technology for the production of energy from organic material present in wastewaters. The current densities that can be generated with laboratory BESs now approach levels that come close to the requirements for practical applications. However, full-scale implementation of bioelectrochemical wastewater treatment is not straightforward because certain microbiological, technological and economic challenges need to be resolved that have not previously been encountered in any other wastewater treatment system. Here, we identify these challenges, provide an overview of their implications for the feasibility of bioelectrochemical wastewater treatment and explore the opportunities for future BESs.