The London Underground (often shortened to the Underground) is a rapid transit system in the United Kingdom, serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex. It incorporates the oldest section of underground railway in the world, which opened in 1863 and now forms part of the Circle, Hammersmith & City, and Metropolitan lines; and the first line to operate electric trains, in 1890, now part of the Northern line.

The Underground system is also colloquially called the Tube. As commonly used today both by Londoners and in most official publicity, this term embraces the entire system. Originally, though, it applied only to the deep-level lines with trains of a smaller and more circular cross-section, and served to distinguish them from the sub-surface "cut-and-cover" lines that were built first and originally used steam locomotives.

The earlier lines of the present London Underground network were built by various private companies. They became part of an integrated transport system in 1933 when the London Passenger Transport Board (LPTB) or London Transport was created. The underground network became a separate entity in 1985, when the UK Government created London Underground Limited (LUL). 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.

London ⁱ/ˈlʌndən/ is the capital city of England and the United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its founding by the Romans, who named it Londinium. London's ancient core, the City of London, largely retains its square-mile mediaeval boundaries. Since at least the 19th century, the name London has also referred to the metropolis developed around this core. The bulk of this conurbation forms the London region and the Greater London administrative area, governed by the elected Mayor of London and the London Assembly.

London is a leading global city, with strengths in the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism and transport all contributing to its prominence. It is the world's leading financial centre alongside New York City and has the fifth-largest city GDP in the world (and the largest in Europe). London has been described as a world cultural capital. It was the world's most visited city during 2011 and London Heathrow is the world's busiest airport by number of international passengers. London's 43 universities form the largest concentration of higher education in Europe. In 2012 London will become the first city to host the modern Summer Olympic Games three times.