Diesel engines in mobile off-highway applications include those used in railway locomotives, marine vessels, farm equipment, construction and industrial equipment, and mobile refrigeration. Engines in these five classes are responsible for about one-third of all mobile diesel emissions (highway vehicles account for nearly all of the rest). Emissions from these engines are not presently subject to regulation. This paper presents estimates of emission factors and nationwide emissions inventories for each of these equipment classes. Estimates of achievable emissions levels, the technologies required to achieve them, and the cost-effectiveness of emissions control for each class are also presented. Emissions test procedures and recommended regulatory approaches for controlling off-highway diesel emissions are discussed as well.