Drivers now have to display a coloured sticker on their vehicle to enter the inner city zones. The colour depends on the pollutants the vehicle emits.
The cities are gradually phasing in fines of 40 euros (�29;$58) for anyone caught driving without a sticker.
Other German cities - but not all - plan to have such zones later in 2008.
The stickers - green, red or yellow - are mandatory not only for locals but also for foreign drivers, including tourists.