Real-world vehicle emissions differ from the legislative emissions limits for a number of reasons. Emissions can be substantially lower but in most cases emissions in real driving conditions are higher than the type-approval values. This is especially the case for NOx emissions from diesels. Between Euro 1 and Euro 5 standards the European NOx limit decreased by a factor of five but real-world NOx emissions have remained more or less constant. High real-world NOx emissions from light and heavy-duty diesels are the main cause of high NO2 concentrations in cities. Some Euro 6 vehicles now show real-world NOx emissions close to the limit, but many vehicle models still exceed the limit by a factor of eight in real-world driving conditions. This report outlines the main reasons for the deviations and discusses the implications for managing air pollution. The European situation is taken as example.