Bioelectrochemical systems (BES) represent an innovative approach that merges renewable energy generation with efficient waste treatment, enabling the transformation of organic waste into electrical energy. This chapter explores the fundamental scientific principles underlying BES, highlighting their functional mechanisms and electrochemical operations. It examines various BES designs and system configurations, highlighting recent advancements that have contributed to improved efficiency. A thorough examination of different BES models showcases their operational versatility. This chapter also examines the utilization of diverse organic waste sources, including agricultural by-products, food industry residues, and municipal wastewater, as feedstocks for electricity generation, with a focus on how factors such as biodegradability influence energy output. Comparative analyses across different substrates provide valuable insights into system effectiveness. Additionally, this chapter outlines the practical applications of BES, including decentralized wastewater treatment, energy recovery, and sustainable waste utilization. A particular focus is placed on electrogenic microbes that facilitate the biodegradation of organic matter and the transfer of electrons. Ultimately, this chapter highlights the integrated advantages of using BES for both power generation and wastewater purification, positioning the technology as a promising tool for promoting circular economic practices and sustainable environmental solutions.