Technology is available to reduce diesel vehicle and engine emissions in a cost-effective way. The ability to use diesel emission reductions for state implementation plan (SIP) and conformity purposes gives states and localities additional incentive to implement diesel retrofit projects. Diesel retrofit technologies reduce pollution from the existing diesel engine fleet by up to 90% for particulate matter (PM), up to 75% for nitrogen oxides (NOx), and up to 90% for volatile organic compounds (VOCs). Many diesel retrofit projects are being successfully implemented around the country. Clean diesel projects already initiated are expected to result in approximately 20,000 tons of particulate matter reduced over the life of the projects, with estimated public health benefits of about $5 billion. The purpose of this document is to provide guidance on quantifying and using emission reductions from on-road and nonroad diesel vehicles, engines, and equipment that have been retrofitted with emission reduction technology. This guidance document describes how to quantify and use reductions of NOx, VOCs, PM2.5, PM10, and carbon monoxide (CO) in ozone, PM2.5, PM10, nitrogen dioxide (NO2), and CO nonattainment and maintenance areas. You can use the emission reductions resulting from implementing a retrofit project in a SIP to help achieve reasonable further progress (RFP), attainment of the national ambient air quality standard (NAAQS or "standard"), or maintenance of the NAAQS; and in transportation conformity and general conformity determinations. This guidance document is updated to reflect the new quantification procedures to use EPA's latest on-road emissions model, the MOtor Vehicle Emissions Simulator (MOVES) model. For nonroad emissions, the nonroad portion of the National MOBILE Inventory Model (NMIM) is the preferred way to estimate benefits of retrofit programs. State and local agencies developing SIPs and conformity analyses for California should consult with EPA Region 9 for information on the current version of EMFAC approved for use in California and for information on how to quantify emission reductions from retrofit projects.