Euro IV, Euro V and Euro VI are emission standards which come in the form of a series of European directives. They define the limits for exhaust emissions of new vehicles sold in the EU.
Emissions of NOx are regulated for most vehicle types, including cars, lorries, trains, tractors and similar machinery, barges, but excluding seagoing ships and aeroplanes. For each vehicle type, different standards apply. Non-compliant vehicles cannot be sold in the EU, but new standards do not apply to vehicles already on the roads. No use of specific technologies is mandated to meet the standards, though available technology is considered when setting the standards.

Early Introduction of Clean Engines.
EU Member States are allowed to use tax incentives in order to speed up the marketing of vehicles meeting new standards ahead of the regulatory deadlines. In Germany, road toll discounts were introduced in 2005 which stimulated early launch of Euro V trucks. Under the Euro VI proposal, incentives can also be given for scrapping existing vehicles or retrofitting them with emission controls.