In 2007, the Energy Independence and Security Act (Public Law 110-140) authorized the U.S. Geological Survey (USGS) to conduct a national assessment of potential geologic storage resources for carbon dioxide (CO₂) in cooperation with the U.S. Environmental Protection Agency and the U.S. Department of Energy. The first year of that activity was specified for development of a methodology to estimate storage potential that could be applied uniformly to geologic formations across the United States. After its release, the methodology was to receive public comment and external expert review. An initial methodology was developed and published in March 2009 (Burruss and others, 2009), and public comments were received. The report was then sent to a panel of experts for external review. The external review report was received by the USGS in December 2009. This report is in response to those external comments and reviews and describes how the previous assessment methodology (Burruss and others, 2009) was revised. The resource that is assessed is the technically accessible storage resource, which is defined as the mass of CO₂ that can be stored in the pore volume of a storage formation. The methodology that is presented in this report is intended to be used for assessments at scales ranging from regional to subbasinal in which storage assessment units are defined on the basis of common geologic and hydrologic characteristics. The methodology does not apply to site-specific evaluation of storage resources or capacity.