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The London Underground is a metro system serving a large part of Greater London and neighbouring areas of Essex, Hertfordshire and Buckinghamshire in the UK. It is the world's oldest underground railway. It was also the first underground railway to operate electric trains. It is usually referred to as the Underground or the Tube—the latter deriving from the shape of the system's deep-bore tunnels—although about 55% of the network is above ground.

The earlier lines of the present London Underground network, which were built by various private companies, became part of an integrated transport system (which excluded the main line railways) in 1933 with the creation of the London Passenger Transport Board (LPTB), more commonly known by its shortened name: "London Transport". The underground network became a single entity when London Underground Limited (LUL) was formed by the UK government in 1985. Since 2003 LUL has been a wholly owned subsidiary of Transport for London (TfL), the statutory corporation responsible for most aspects of the transport system in Greater London, which is run by a board and a commissioner appointed by the Mayor of London.

The Underground has 270 stations and approximately 400 km (250 miles) of track, making it the longest metro system in the world by route length, and one of the most served in terms of stations. In 2007, over one billion passenger journeys were recorded.

The tube map, with its schematic non-geographical layout and colour-coded lines, is considered a design classic, and many other transport maps worldwide have been influenced by it.