A system designed to address the problem of distribution system monitoring is described in this paper. The developed system employs an array of common analytical instrumentation, such as pH and chlorine monitors, coupled with advanced interpretive algorithms in an event monitor to provide detection/identification-response networks that are capable of enhancing system security, as has been advocated by several Federal research initiatives. Through the use of laboratory testing, pilot scale testing on pipe loops, and real world beta site deployment, the system has been shown to be effective in detecting a wide diversity of possible threats. The system has been challenged with, and found effective against, a variety of agents including TIMs (toxic industrial materials), TICs (toxic industrial compounds), chemical warfare agents, and biological warfare materials. Other possible more obscure classes of threat agents such as street drugs, homemade toxins and commercial preparations have also been tested. In addition, the system has been shown to recognize common accidental intrusions such as antifreeze and sewage. The response of these various agents is not only adequate to detect the presence of a contaminant, but the unique profile of the responses allow for some degree of identification. Through the use of a searchable library of agent profiles the system is capable of providing not only an alarm but also an identification of the likely cause. The profiles of over 60 of the most likely threat agents and many common contaminants have been compiled. A proprietary baseline estimator dramatically and immediately reduces false warnings from regular fluctuations in operational parameters upon start-up. As time since deployment increases the number of false positives is rapidly reduced to near zero by the system's programmed ability to learn what is normal for a given operation. The rapid detection and identification of breaches of security in the water distribution system is crucial in initiating appropriate corrective action. The ability of the described system to detect incursion on a real time basis and give indications as to the cause could dramatically reduce the impact of any such scenario. As the vulnerability of the distribution system becomes more widely recognized, the deployment of a system such as the one described in this paper will be an invaluable tool in maintaining the integrity of the nations drinking water supply. Includes 12 references, tables, figures.