Disinfection of drinking water has been used for many years for the prevention of waterborne disease through inactivation of pathogenic microorganisms. Concerns about waterborne disease outbreaks caused by the protozoan Giardia lamblia, the cysts of which are very resistant to inactivation, and the need to more precisely control the disinfection process to reduce formation of potentially hazardous chemical byproducts have made it necessary to more closely examine the biocidal aspects of drinking water disinfection. The use of mathematical concepts based on microbial inactivation kinetics to predict disinfectant concentration and contact times needed to inactivate specific proportions of various pathogens and indicator species has received increasing attention as an approach to better control of the disinfection process. The report describes the use of these concepts, the problems in extrapolation from the available data and in application of laboratory disinfection data to field conditions.