The Emission Factors and Inventory Group (EFIG) of the U.S. Environmental Protection Agency (EPA) is responsible for compiling and maintaining national emission data for the criteria and hazardous air pollutants. To promote the consolidation of criteria and toxic pollutant data in one national inventory, in 1999 EFIG combined the National Emission Trends (NET) criteria air pollutant inventory, and the National Toxics Inventory (NTI) into one 'National Emission Inventory.' The procedures EPA applies to prepare criteria air pollutant emissions (plus ammonia) for the NEI are documented separately from the procedures EPA uses to prepare hazardous air pollutant emissions for the NEI. It is expected that in the future, the methodology descriptions for developing both the criteria and toxic emissions data, will be consolidated into one document. This document includes methodologies for estimating emissions , 1985-1999, for the following criteria pollutants: carbon monoxide (CO), nitrogen oxides (NOx), lead (Pb), particulate matter less than 10 and 2.5 microns in diameter (PM-10/PM-2.5), sulfur dioxide (SO2), volatile organic compounds (VOC), and ammonia (NH3). This document does not include the data results, only method descriptions.