In 2015, the Volkswagen company admitted it had manipulated about eleven million cars worldwide in order to hide the fact that the emissions of nitrogen oxides exceeded the legal limit. Subsequent investigations by the public authorities revealed that cars from other manufacturers had also been modified for the same purpose. The present contribution describes and assesses the steps taken by public authorities, in particular in the United States, by the EU and in some EU Member States, to ensure and enforce compliance with existing car emission ceilings. It identifies deficiencies in the enforcement system and suggests ways to improve the present situation within the EU.