There is increased interest among federal and state regulatory agencies and water utilities in the factors that result in water quality deterioration between the treatment plant and the consumer' tap. This has resulted in a number of efforts being made to develop models that predict the propagation of contaminants in distribution systems. The U.S. EPA, in cooperation with the North Penn Water Authority has developed and field tested such a model. The model has undergone extensive field testing. In addition several continuous samplers have been tested and evaluated. The EPA model was used in a recent waterborne disease outbreak study in Cabool, Missouri, in which the U.S. EPA and CDC collaborated. The model was instrumental in assisting the CDC to reach the conclusion that the outbreak was in fact waterborne and probably due to a waterborne contaminant that was introduced, possibly from sewer overflows that occurred at the same time as two line breaks in the distribution system.