Diesels are the most efficient IC engines. The excellent combustion emits the least CO2, HC and CO. But emissions of NOx and ultrafine soot particles are inherently high. No problem: exhaust after-treatment diminishes NOx and eliminates solid par-icles. This is implemented in all modern engines. But what about older engines? These are often deployed for 20 and more years, e.g. trucks, construction machines, locomotives, and gensets. Such engines must be retrofitted. Otherwise we cannot alleviate the deteriorating air quality and respiratory distress in our cities. DPF facilitate mobility combined with clean urban air. And DPF are a big step to in-hibit global warming due to atmospheric black carbon particles. This opens a large and very meaningful new market. A big challenge and a great opportunity for ongoing technology advances in engines, fuels and exhaust-gas after-treatment. This book contains contributions from pioneering innovative engineers. They devel-oped the retrofit technology and are now motivated to make it a technical standard worldwide.