Maintaining chlorine and chloramine residual is a key factor that determines potability of distributed water. Decay of these disinfectants is affected by various water quality and infrastructure related factors. Understanding the decay of residuals in the distribution system is critical with respect to safe delivery of drinking water and in developing water quality models that can be used by utilities for the operation of potable water delivery systems. In order to comply with microbial and disinfection byproduct (M/DBP) regulations, water suppliers are continually investing in treatment processes for achieving greater levels of inactivation and DBP precursor removal. As a result of these changes in water quality, maintaining a disinfectant residual in the distribution system can be challenging Bench-scale testing has been used to determine the effect of advanced treatment (enhanced coagulation, granular activated carbon adsorption, ozonation, biofiltration, and ultraviolet irradiation) on chlorine and chloramine decay. Additional studies examine the effects of disinfectant dose, temperature, pH, blending, and rechlorination. The effects of water quality on pipe-wall demand were evaluated using the distribution system simulator at the U.S. Environmental Protection Agency's Test and Evaluation Center. Several models are developed regarding disinfectant decay, making it easier to predict chlorine demand after changes in treatment. The multi-species water quality analysis and network path analysis extensions to EPANET will make the modeling package more robust and will allow utilities to predict chlorine residual at any point in their distribution system with greater confidence.