Diesel engines play a vital role in key industry sectors such as goods movement, public transportation, construction, and agriculture. A unique combination of efficiency, power, reliability, and durability make diesel the technology of choice for these sectors. However, the durability of the technology does not lend itself to rapid fleet turnover and investment in new equipment that meets more stringent environmental standards. Because of this, the full air quality benefits of the very stringent new engine emission standards in the US2007 Diesel Rule ("Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements.") and the Nonroad Diesel Rule ("Clean Air Nonroad Diesel Rule.") will likely take decades to achieve. Further, the regulatory authority of EPA and states to address the existing fleet of over 11 million diesel engines is rather limited. In response, EPA began the Voluntary Diesel Retrofit Program in 2000 to discuss broad initiatives to modernize and upgrade (i.e., retrofit) current engines with modern emission control equipment or to accelerate the replacement of these engines with newer ones. Given the diversity of applications and engines, as well as significant technical and funding issues, the Clean Diesel Retrofit Work Group was formed in 2004 under the auspices of the EPA Clean Air Act Advisory Committee (CAAAC) to advise EPA on how best to expand the initiative.This report is the culmination of the work of the Clean Diesel and Retrofit Work Group since April 2004. It provides consensus-based recommendations as well as other recommendations. Some recommendations are sector-specific; others apply more broadly. It is our hope that this report will substantially further our Nation's efforts to achieve healthy air for its citizens.