This chapter reviews the state-of-the-art use of microbial fuel cells (MFCs) as sensors for the online monitoring of water quality. This is a very promising application of MFC technology that has not yet been fully explored. The most intuitive analytical application of MFCs is with regards to the monitoring of the labile organic carbon content in wastewaters. This is because, within a specific range dictated by the Monod equation, the amount of electrons produced by the anode biofilm is a direct function of the concentration of biodegradable organic compounds in the feeding water. The literature on the use of MFCs as a tool to measure the chemical and biological oxygen demand in wastewaters is therefore well established and includes also in-field validations.Much more recently, there has been an increasing interest in exploring the use of MFCs also as a means to detect toxicants in water, which is encouraged by the great advantages of this technology over traditional analytical methodologies. MFCs can in fact operate in the field and online and have the great benefits of simplicity, higher stability compared to enzyme-based sensors, and depending on the material used, cost-effectiveness. The very few proof-of-concept studies on this topic are very promising. The presence of a bioactive toxicant in the feed affects the performance of the electroactive biofilm at the anode and causes a change in the MFC outputs that is easily detected. The full possibilities that MFCs open in the field of water quality monitoring have to be, however, better explored. In particular, some challenges, such as sensitivity and selectivity, remain still to be addressed.