MOBILE5 is a computer program that estimates hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) emission factors for gasoline-fueled and diesel highway motor vehicles. MOBILE5 calculates emission factors for eight individual vehicle types in two regions (low-and high-altitude) of the country. MOBILE5 emission factor estimates depend on various conditions such as ambient temperatures, average travel speed, operating modes, fuel volatility, and mileage accrual rates. Many of the variables affecting vehicle emissions can be specified by the user. MOBILE5 will estimate emission fators for any calendar year between 1960 and 2020, inclusive. The 25 most recent model years are considered to be in operation in each calendar year. MOBILE5 supersedes MOBILE4.1, and is to be used by the States in the preparation of the highway mobile source portion of the 1990 base year emission inventories required by the Clean Air Act Amendments of 1990. Compared to MOBILE4.1, MOBILE5 incorporates several new options, calculating methodologies, emission factor estimates, emission control regulations, and internal program designs.