Terrorist attacks have heightened concern about intentional threats to the U.S. water system, whether from physical destruction, computer interference, or chemical, microbial, or radioactive contamination. Such intentional contamination events can have a profound impact on public health and confidence in the nation's water infrastructure. An Early Warning System (EWS) can be an important tool to avoid or mitigate the impacts of an intentional contamination event in time to allow an effective local response that reduces or eliminates adverse impacts (ILSI, 1999). An integrated EWS includes sensors to detect the contaminant; systems to transmit, compile, and analyze data; links for communication and notification; and protocols for decision making and emergency response. The goal of this EWS document is to review the state-of-the-art technologies and techniques for integrated EWSs for drinking water infrastructure, particularly for finished water supplies and distribution systems. The report summarizes and evaluates current and emerging EWS technologies for identifying general categories of chemical, microbial, and radiological contaminants. It also identifies future directions, technical issues, and research gaps. Information was gathered from a variety of sources, including company information, government information, verification studies, field case studies, and expert opinions.