The paper discusses the National Emissions Data System (NEDS) Mobile Source methodology, the State Implementation Plan (SIP) Mobile Source methodology, differences and problems between the two methodologies, and the research program that will result in an improved Mobile Source methodology that will serve both the NEDS and SIP emission inventory objectives. The methodologies developed by the research program will reside in the Area and Mobile Source (AMS) Subsystem of the Aerometric Information Retrieval System (AIRS). EPA annually estimates emissions of the five criteria pollutants--particulates (TSP), SO2, NOx, reactive volatile organic compounds (VOCs), and CO. Historically, these emission estimates were stored in NEDS as point or area source emissions. Area sources usually include all mobile sources and any stationary sources to small, difficult, or numerous to classify as point sources. In NEDS, an area source is defined as a source that emits < 100 tons/year of a criteria pollutant. Areas that are in non-attainment with the National Ambient Air Quality Standards (NAAQS) are required to develop SIPs for use in the demonstration of progress toward meeting these standards. The major objectives of AIRS include providing storage and reporting capability for air quality, emissions, and enforcement data required by EPA regulations and the Clean Air Act.