Reducing emissions from diesel engines is one of the most important public health challenges facing the country. Despite EPA's stringent diesel engine and fuel standards taking effect over the next decade, the 20 million engines already in use will continue to emit large amounts of nitrogen oxides (NOX) and particulate matter (PM)--both of which will contribute to serious public health problems for years to come. Fortunately, a variety of cost-effective technologies can dramatically reduce harmful emissions, save fuel, and help our nation meet its clean air and sustainability goals. To meet these challenges, the U.S. Environmental Protection Agency (EPA) established the National Clean Diesel Campaign (NCDC). NCDC consists of both regulatory programs to address new engines and innovative nonregulatory programs to address the millions of diesel engines already in use.