In April 2003, the U.S. Environmental Protection Agency (EPA) published the Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries (Regional Criteria Guidance) in cooperation with and on behalf of the six watershed statesNew York, Pennsylvania, Maryland, Delaware, Virginia, and West Virginiaand the District of Columbia. The culmination of three years of work, the criteria document resulted directly from the collective contributions of hundreds of regional scientists, technical staff, and agency managers as well as the independent review by recognized scientific experts across the country (U.S. EPA 2003). This document represents the third formal addendum to the 2003 Chesapeake Bay water quality criteria document. As such, readers should regard the sections in this document as new or replacement chapters and appendices to the original published report and two prior published addendums. The criteria attainment assessment procedures published in this addendum replace and otherwise supercede similar criteria assessment procedures originally published in the 2003 Regional Criteria Guidance and the 2004 and 2007 addendums (U.S. EPA 2003, 2004, 2007). Publication of future addendums by EPA in coordination with and on behalf of the Chesapeake Bay Program watershed jurisdictional partners is likely. Continued scientific research and management applications reveal new insights and knowledge that should be incorporated into revisions of state water quality standards regulations in upcoming triennial reviews.