Recent concerns over the potential for bioterrorism and infrastructure resiliency have stimulated the development of sensors for real-time monitoring of biological contaminants in water distribution systems. Private and government water utilities are responding to these concerns and putting in place commercially available sensors. These sensors need to be evaluated independently (without bias) for optimization of their use, evaluation of their claimed capabilities, and their potential for integration into a supervisory control and data acquisition (SCADA) system. This research effort evaluated real-time sensors for their ability to monitor microbial contaminants in water distribution systems through rigorous evaluation protocols in afield scale test-bed laboratory specifically constructed for this purpose. Water quality sensors including the HACH Guardian Blue Monitoring Platform; the JMAR BioSentry unit; the S::CAN spectro::lyser technology; and the GE 5310 total organic carbon unit were evaluated using intrusion of Escherichia coli (E. coli) to determine the sensor's response to a microbial contaminant. Detection capabilities were examined with the contaminating E. coli bacteria suspended in deionized water or tap water with concentrations ranging from 10 3 to 10 6 cfu/mL. Most sensors were responsive to an increase in E. coli concentrations with an observed useful dynamic range between 10 3 cfu/mL to 10 6 cfu/mL. Below this range, sensors provided signals not distinguishable from noise while exceeding the threshold caused saturation and mis-classification by some sensors. The data effectively shows that select sensors can detect microbial water quality changes and thus be utilized in part of an early warning system for monitoring intrusion events in water distribution systems.