In order to improve air quality in Europe the pollutant exhaust emissions of road vehicles are limited by regulations. The regulatory limits apply to exhaust emission measurements during the vehicle type approval procedure. The limit values were lowered several times over the last decades from EURO 0 to EURO 3 for heavy duty vehicles and EURO 4 for cars and light duty vehicles. Further steps have already been defined for the coming years (EURO 4 and 5) or are under discussion (EURO 6). Air quality, however, has not improved as much as predicted with the tightening of emissions standards for new vehicles, especially in respect of nitrogen oxides (NOx). One probable reason for this is the gap between the performance of emission control strategies and measures during type approval tests and their effectiveness under real operating conditions. This study aims to provide explanations for the gap, both for cars and heavy duty vehicles (HDVs) on the basis of a comparison of operating conditions under test cycles and operating conditions in real traffic. Possibilities to improve test cycles to match onroad conditions more closely are examined and recommendations made for the introduction of in-use control tests. The study focusses mainly on NOx emissions and heavy duty vehicles