Following September 11, 2001, the threat of deliberate contamination of potable water became a reality. An analysis of the components of a water utility will reveal that the distribution system is quite vulnerable to intentional contamination. Dilution limits the effectiveness of source water contamination, and the treatment facility is designed to remove harmful microbial and chemical contaminants from the water. However, the only method of contaminant control in the distribution system is maintaining disinfectant residual and pressure. Focus has been placed on monitoring of traditional water quality characteristics of chlorine residual, pH, conductivity, oxidation-reduction potential, and turbidity as an indicator to contamination; but, very little has been done to determine the sensitivity of these instruments for potential chemical, biological, and radiological contaminants. It is not known if the parameter measured will detect the contaminant of concern. Furthermore, it not established that monitoring of these characteristics in the distribution system could provide indication of intentional contamination. Water quality variability in source water, finished water, and the distribution system may exceed the change associated with the addition of a contaminant. The implementation of distribution system monitoring for contaminant detection should be incorporated into a utility's emergency response plan. One of the most important steps is the analysis of data to develop a set of normal operating conditions from which a decision can be made to determine if the water has been contaminated. If a monitoring technology did detect a contamination event, quick action would be required to confirm the contamination and put in place mitigation measures to prevent movement and consumption of the contaminant. Alternatively, false positives could create substantial problems for a utility. These considerations must be carefully reviewed before a utility decides to install distribution system monitoring for the purpose of contaminant detection. This paper discusses over 30 potential biological and chemical contaminants, the lethal or infective dose, chemical or biological properties, and evaluates the traditional online monitoring technologies that measure disinfectant residual, pH, conductivity, oxidation-reduction potential, and turbidity for their ability to detect lethal or infective concentrations of a contaminant. Finally, the efficacy of these technologies in regard to a utility's emergency response plan is determined. Includes tables.