The London Underground (also known simply as the Underground or by its nickname the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England.

The Underground has its origins in the Metropolitan Railway, opening on 10 January 1863 as the world's first underground passenger railway. It is now part of the Circle, District, Hammersmith & City and Metropolitan lines. The first line to operate underground electric traction trains, the City & South London Railway in 1890, is now part of the Northern line.

The network has expanded to 11 lines with 250 miles (400 km) of track. However, the Underground does not cover most southern parts of Greater London; there are only 33 Underground stations south of the River Thames. The system's 272 stations collectively accommodate up to 5 million passenger journeys a day. In 2020/21 it was used for 296 million passenger journeys, making it one of the world's busiest metro systems.

The system's first tunnels were built just below the ground, using the cut-and-cover method; later, smaller, roughly circular tunnels—which gave rise to its nickname, the Tube—were dug through at a deeper level. Despite its name, only 45% of the system is under the ground: much of the network in the outer environs of London is on the surface.