The Environmental Protection Agency's (EPA) National Clean Diesel Campaign (NCDC) is a comprehensive initiative to reduce pollution from diesel engines throughout the country, including vehicles on highways, city streets, construction sites, and ports. The NCDC comprises both regulatory programs to address new engines and voluntary programs to address the millions of diesel engines already in use. On the regulatory side, EPA is successfully implementing emissions standards for engines in the 2007 Heavy-Duty Highway Engine Rule and the Tier 4 Nonroad Rule and developing new emission requirements for locomotives and marine diesel engines, including large commercial marine engines. On the voluntary side, EPA is addressing engines that are already in use by promoting a variety of innovative emission reduction strategies such as retrofitting, repairing, replacing and repowering engines; reducing idling; and switching to cleaner fuels. The voluntary programs are accomplished in partnership with state and local governments, environmental groups and industry. The emissions standards for new engines will reduce both highway and nonroad engine emissions by roughly 90%. However, these emission reductions occur over a long period of time as new engines are phased into the fleet. Retrofitting diesel engines currently in use will allow significant and immediate emission reductions from diesel engines that would not otherwise be addressed. The purpose of this technical analysis is to evaluate the cost effectiveness of retrofitting existing heavy-duty diesel engines to reduce particulate matter (PM). (The cost effectiveness of the regulatory measures EPA has implemented is addressed the rulemakings.) Analysts in EPA's Office of Transportation and Air Quality (OTAQ) evaluated the costs and emissions benefits of retrofitting school buses, freight trucks, and bulldozers with diesel oxidation catalysts (DOCs) and catalyzed diesel particulate filters (CDPFs), two of the most common PM emissions reduction technologies for diesel engines.