The United States Environmental Protection Agency's (EPA's) Office of Water has developed this Thesaurus of Microbiological Risk Assessment (MRA) Terms because it is important for risk assessors, managers, and communicators to have available and to use common, understandable terms and definitions for the various facets of the MRA process. This Thesaurus is a collection of definitions of terms that may be relevant for microbial risk assessment (MRA). It should help risk assessors, managers, and communicators become aware of various definitions so that they can better communicate with each other and avoid misunderstandings. It should also help increase transparency and contribute to a common understanding of the MRA process and presentation of MRA results.