The on-going development of a National Primary Drinking Water Regulation to control microbial contamination in groundwater-based drinking water systems (Groundwater Disinfection Rule), requires an adequate understanding of a wide array of scientific and technical issues. Among the first topics taken up in the discussions on this rule was the determination of the necessary data and technologies to successfully specify and implement appropriate regulatory elements. A research plan was developed early in 1995 and projects initiated. The plan has been reviewed and modified as results accumulate; new projects have been initiated as necessary. This paper discusses research conducted in five principal areas: definition of the public health problem associated with microbial contamination; factors affecting and limiting microbial contamination of groundwater sources; system protection and treatment approaches; enhanced monitoring approaches to ensure public protection from waterborne viruses and bacteria; and, the regulatory costs and public health benefits of new regulations.