The United States Environmental Protection Agency (EPA or Agency) Office of Ground Water and Drinking Water (OGWDW) is developing interrelated drinking water regulations to control microbial pathogens and disinfectants/disinfection byproducts in drinking water. These rules are collectively known as the microbial/disinfection byproducts (M-DBP) rules. The Safe Drinking Water Act (SDWA) Amendments of 1996 require EPA to develop rules to balance the public health risks from pathogens and DBPs. The Stage 1 Disinfectants and Disinfection Byproducts Rule (Stage 1 DBPR) and the Interim Enhanced Surface Water Treatment Rule (IESWTR), the first set of M-DBP rules under the SDWA Amendments, were promulgated in December 1998. The Stage 1 DBPR and the IESWTR were the culmination of a 6-year (1992-1998) rule development process that included regulatory negotiations with representatives of the water industry, environmental and public health groups, and local, State, and Federal government agencies. The Amendments also require EPA to publish a Stage 2 DBPR. To support rule development, EPA expanded its microbial and DBP research program and entered into collaborative efforts with other agencies and the water industry to collect data. This data collection effort included the Information Collection Rule (ICR) and the ICR Supplemental Surveys (ICRSS). In addition, under a joint effort between EPA and the National Rural Water Association (NRWA), NRWA state chapters conducted a survey of byproduct and treatment information at small public water systems (PWSs). The purpose of the ICR was to provide microbial and DBP occurrence and treatment information from large PWSs to support M-DBP regulations. EPA has worked with stakeholders under the Federal Advisory Committee Act (FACA) to develop the Stage 2 DBPR and Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). These rules are being developed concurrently, using occurrence data from the ICR and other available sources, to ensure that microbial protection is maintained or enhanced while exposure to DBPs is reduced. This occurrence and exposure assessment provides background information for the LT2ESWTR.