Under the Surface Water Treatment Rule (SWTR), systems are required to use CT, or contact time, to demonstrate the adequacy of disinfection. This paper explains what CT is (concentration multiplied by contact time), how systems can determine how much inactivation they are achieving by its application, why the US Environmental Protection Agency (USEPA) adopted the CT concept, and how systems can determine whether or not they comply with the level of disinfection required under the SWTR. A number of tables and figures are included. One table gives expected removals by filtration and minimum recommended levels of inactivation; a series of tables lists the USEPA's calculated CT values for inactivating Giardia with free chlorine at a variety of temperatures. A table and several figures describe baffling conditions.