A Low Emission Zone (LEZ) was introduced in Brussels in January. After a transitionary period, it is compulsory to register foreign vehicles in order to gain access.
Number plates are recorded when entering the city, and if a vehicle is too old, a fine of €350 will be issued. Currently, only vehicles up to Euro 1 standards are excluded, but a charge of €150 is made for non-registration.
Relevant information and regulations can be found on the Green-Zones portal, where LEZ registration may also be made.