The London Underground is a rapid transit system that serves a large part of Greater London and some neighbouring areas of Essex, Hertfordshire and Buckinghamshire. It is the world's oldest underground railway system. Services began on 10 January 1863 on the Metropolitan Railway; most of the initial route is now part of the Hammersmith & City line. Despite its name, about 55% of the network is above ground. Popular local names include the Underground and, more colloquially, the Tube, in reference to the tubular cylindrical shape of the system's deep-bore tunnels.

The Underground has 268 stations and runs over approximately 250 miles (400 km) of line According to " ", the total route length is 253 mi or 408 km. But blank">in July 2007 the same page showed the same route length even though there were more stations. Thus it must not have been fully updated for the closure of the East London line, whose route length was about 7 km. , making it the longest underground railway in the world by route length, and one of the most served in terms of stations. There are also numerous closed stations. In 2005, 971 million passengers used the Underground and in 2007, for the first time ever, over one billion passengers were recorded. As of March 2007, just over 3 million passengers use the Underground each day, with an average of 3.4 million passengers on weekdays.

Since 2003, the Underground has been part of _Transport for London (TfL), which also administers numerous other transport-related functions, including the famous red double-decker buses. The former London Underground Limited was a subsidiary of London Regional Transport, a statutory corporation.