This document is the User's Guide To MOBILE3. MOBILE3 is a computer program that calculates emission factors for hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) from highway motor vehicles. The program uses the calculation procedures and emission factors presented in Compilation of Air Pollutant Emission Factors: Highway Sources, July 1984 (EPA 460/3-84-005). MOBILE3 calculates emission factors for eight individual vehicle types in two regions of the country. MOBILE3 emission estimates depend on various ambient, vehicle usage, and local conditions such as temperature, speed, mileage accumulation and accrual distributions, vehicle-miles-traveled distributions, etc. MOBILE3 will estimate emission factors for any calendar year between 1970 and 2020, inclusive. The 20 most recent model years are considered in operation in each calendar year. MOBILE3 supercedes MOBILE2. Compared to MOBILE2, MOBILE3 incorporates several new options, calculating methodologies, emission factor estimates, emission control regulations, and internal program designs. As such, MOBILE3 should be easier to maintain, transport, alter and comprehend.