Based on the functioning of diesel and gasoline engines, the engine-specific components such as NOx storage catalysts, diesel particulate filters, oxidation catalysts in diesel engines, or lambda sensors with three-way catalysts in gasoline engines are explained in terms of their function and effect. A detailed section is dedicated to the current exhaust emission regulations for passenger cars and commercial vehicles. The influence of electronic diesel control and electronic control of gasoline engines on exhaust composition is explained. Detailed sections on on-board diagnostics for diesel and gasoline engines complete the content and provide the necessary background information for all exhaust-related topics.