WaterSentinel (WS) serves as a demonstration project, or pilot, for designing and implementing an effective contamination warning system (CWS) in a drinking water distribution system. As part of WS, a CWS should be developed that encompasses monitoring technologies and detection strategies, combined with enhanced public health surveillance to collect, integrate, analyze, and communicate information to provide a timely warning of potential water contamination incidents and initiate response actions to minimize public health and economic impacts. The success of a CWS, and hence WS, depends on the ability to effectively integrate these components and analyze the resulting information in a timely manner to inform response actions that can substantially reduce the potential consequences of a contamination incident. Current means of monitoring water quality parameters, such as periodic grab sampling for disinfectant residual and infrequent sampling and analysis for a small number of specific contaminants, may be of limited scope and usefulness in the WS-CWS. Therefore, the WS-CWS aims to detect contaminants by utilizing a network of online water quality sensors, deployed throughout a drinking water distribution system, that are responsive to many contaminants. In many cases, information from online water quality sensors should provide the first indication of possible contamination, and should set into motion response actions to either corroborate or rule out contamination. Establishing which water quality parameters provide the broadest coverage and most reliable indication of contamination is of critical importance to the success of this component of the WS-CWS. Research, conducted by online water quality sensor manufacturers and the EPA provided information about the potential of various contaminants of concern to produce detectable changes in specific water quality parameters. While there are numerous parameters that respond to contamination, a literature review found that the most effective parameters for detecting the 33 WS Baseline contaminants are free chlorine, total organic carbon (TOC), conductivity and pH. Oxidation/reduction potential corroborates chlorine sensor results. Other parameters such as chloride, nitrate and ammonia have been observed to change in the presence of contaminants but mostly due to interference of concomitant ions. Turbidity, which can be highly variable, is not a good primary indicator of contamination. Utilizing online water quality monitoring as an indicator of drinking water contamination should be an integral part of the WS system architecture. Online monitoring should enable water utilities to detect potential contamination quickly and launch an appropriate response. Future work on online monitoring of radionuclides, a broader range of chemical and biological agents, and the use of event detection software should improve detection as the WS-CWS evolves.