This document, entitled “Microbial Source Tracking Guide Document”, was published in June 2005 by the National Risk Management Research Laboratory of the U.S. Environmental Protection Agency. The guide aims to provide readers with insights into various tools and approaches used to track sources of fecal contamination impacting water quality in streams, rivers, lakes, and marine beaches. Descriptions of research and several case studies gathered through workshops, literature searches, and phone interviews are also provided.