The A406 or the North Circular Road is a road which crosses North London, UK, linking West and East London. The speed limit is either 30 or 50 mph depending on the stretch of the road, including the sections with multiple carriageways. It, together with the South Circular Road, forms a ring road through the inner part of Outer London. This ring road does not make a complete circuit of the city; being C-shaped rather than a complete loop as the crossing of the River Thames in the east is made on the Woolwich Ferry.

In the west, the A406 begins at junction 1 of the M4 Motorway at Gunnersbury, from which the South Circular Road (A205) heads South over Kew Bridge. The A406 runs North under the name "Gunnersbury Avenue". Whereas most of the South Circular goes by the names of the roads widened to accommodate it, the North Circular is typically called "North Circular Road". This is because the A406 has been subject to much more road improvement, often coupled with demolition of existing houses and urban infrastructure, hence the link with the old roads has been lost. In the West where the road made slowest progress and is now unlikely ever to be further widened, the original names are preserved.