The primary objective of the research at the US Environmental Protection Agency (USEPA) Water Awareness Technology Evaluation Research Security (WATERS) Center is to develop and evaluate performance criteria for on-line real-time drinking water monitoring instrumentation. A secondary objective of this research is to develop a matrix of surrogates for evaluating and testing the performance of the on-line instruments in a distribution system. The USEPA's Water Awareness Technology Evaluation Research and Security WATERS Center utilizes an above ground simulated distribution system (DSS). Experiments are underway to evaluate the selected on-line sensors in the DSS for their ability to detect changes in water quality due to chemical, physical and microbial agents at concentrations that would present a public risk in a distribution system. Sensitivity, response, limit of detection, reproducibility, and potential for false positives, false negatives and other limitations of the selected sensors are being evaluated. USEPA investigators selected a platform array of real-time on-line sensors and instrumentation that represent the various types of technologies currently used by utilities to monitor water quality. USEPA is conducting studies to evaluate the changes of water quality through the introduction of various contaminants. This paper provides results of the evaluation of existing sensors for their sensitivity and ability to identify changes in water quality due to various physical, chemical, and biological contaminants. Includes tables.