This report documents the development of the 1980 National Acid Precipitation Assessment Program (NAPAP) Emissions Inventory. The current version of the annual inventory, Version 5.0, and the related Version 5.2 Eulerian Modeling Inventory and Version 5.3 Regional Oxidant Modeling Inventory represent the most comprehensive, highest quality emissions data available for the 1980 base year. The inventory spans the 48 contiguous states and 10 Canadian provinces, providing detailed point source data for over 14,000 plants and area source information by source category for 3,070 U.S. counties. Canadian area source data are reported at the province level. Emissions from 11 pollutants (SOb2s, SOb4s, NO[subscript x], Pb, CO, HCl, HF, NHb3s, TSP, VOC, and total hydrocarbons) are included in Version 5.0 of the inventory, while in the 5.2 modeling inventory, pollutants are disaggregated into 39 individual classes. NAPAP Version 5.0/5.2 emissions of SOb2s, NO[subscript x], and VOC are 32.1, 24.4, and 26.2 million tons/year, respectively. Summaries of emissions data are presented at various levels of aggregation including nation, EPA region, state/province, and source category. Emissions data are also analyzed by plant size, stack height, and general source type, and emissions variations are examined on seasonal and daily bases. The Version 5.0 NAPAP Emissions Inventory represents the last of a series of NAPAP inventory compilation efforts for base year 1980. This report provides a history of the inventory development, focusing on enhancements made from version to version. It also includes a number of topics specific to development of Versions 5.0 and 5.2, including development of emission factors for newly added pollutants; incorporation of updated utility data with information from the NAPAP Utility Reference File (NURF); development of temporal, spatial, and species allocation factors; and creation of the Flexible Regional Emissions Data System, used to process the 5.0 inventory into the 5.2 and 5.3 modeling inventories.