Bio-electrochemical systems (BESs) as an energy-efficient wastewater treatment technology have attracted increasing interest in the past decade. A BES is a bio-electrochemical reactor capable for conversion of chemical energy present in the chemical bonds of organic compounds directly to other usable forms of chemical energy or electrical energy by using catalytic reactions initiated by micro-organisms in anaerobic conditions. Energy crisis in recent time has attracted researcher’s interest in BESs as an option to produce electric power or other forms of energy from biomass without producing any carbon emission to the environment. BESs can also be utilized at wastewater treatment facilities to treat different types of organic matter present in the wastewater and recover valuable products, thus ensuring economic sustainability. This chapter comprehensively reviews BES with emphasis on their applications for wastewater treatment. It is anticipated that insights offered in this chapter would facilitate for the use of BES as an energy-efficient wastewater treatment option coupled with direct electricity or recovery of resources. © 2018, Springer Nature Singapore Pte Ltd.