This document is the User's Guide to MOBILE2. MOBILE2 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 Mobile Sources, March 1981 (EPA 460/3-81-005). MOBILE2 calculates emission factors for eight individual vehicle types in three regions of the country. MOBILE2 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. MOBILE2 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. MOBILE2 supersedes MOBILE1. Compared to MOBILE1, MOBILE2 incorporates several new options, calculating methodologies, emission factor estimates, emission control regulations, and internal program designs, As such, MOBILE2 should be easier to maintain, transport, alter and comprehend.