Emissions from nonroad mobile sources can present significant health andenvironmental hazards. The U.S. Environmental Protection Agency (EPA) projects that emissions from these sources will decrease in some categories but increase inothers. As such, we examined EPA's efforts to reduce nonroad mobile source emissions, opportunities for additional reductions, and challenges to addressingnonroad emissions problems. Nonroad mobile sources include marine vessels, locomotives, aircraft, farm and construction machinery, lawn and garden equipment, recreational vehicles, and outdoor power equipment. Nonroad mobile sources produce particulate matter and ozone-forming nitrogen oxides and volatile organic compound emissions, as well as toxic air pollutants, which contribute to a host of health and environmental hazards.