In the USA chlorine is used extensively to inactivate pathogens in drinking water. Beyond this fundamental goal, the practice of chlorination involves maintaining a disinfectant residual in the distribution system to minimise biofilm formation and as an additional safeguard against pathogen intrusion. In contrast to many European countries the disinfectant residuals typically maintained in the distribution system are relatively high. The primary factors that are driving high disinfectant residuals include regulation, and using chlorine as a primary disinfectant rather than other advanced treatment systems. Lower disinfectant residuals are likely in the future with changes in regulations, installation of advanced treatment processes, and improvements in the distribution system.