A review of current literature indicates that Giardia lamblia cysts can be inactivated by chlorine and ozone. Since cysts are more resistant to these disinfectants than most bacteria, higher disinfectant dosages and contact times than those currently used to kill bacteria must be used to inactivate cysts. Although ozone is found to be m ore effective than chlorine, a dual ozone-chlorine disinfection system offers maximum protection against cysts and provides a residual disinfectant in the water supply distribution system. However, prior to actual use, concerns associated with high levels of chlorination (i.e., taste and odor problems, THM formation and pH depression) must be addressed and mitigated. In addition, the hydraulic residence time distribution of each disinfectant application point should be considered when applying recommended CT values obtained from laboratory batch reactor studies.