The Circle line is a spiral-shaped London Underground line, running from Hammersmith in the west to Edgware Road and then looping around central London back to Edgware Road. The railway is below ground in the central section and on the loop east of Paddington. Unlike London's deep-level lines, the Circle line tunnels are just below the surface and are of similar size to those on British main lines. Printed in yellow on the Tube map, the 17-mile (27 km) line serves 36 stations (this includes the 2 Paddington stations!), including most of London's main line termini. Almost all of the route, and all the stations, are shared with one or more of the three other sub-surface lines, namely the District, Hammersmith & City and Metropolitan lines. On the Circle and Hammersmith & City lines combined.

The line's depot is at Hammersmith, close to Hammersmith station, originally built by the Great Western Railway to be operated by the Metropolitan Railway when the joint Hammersmith & City Railway was electrified in the early 20th century. Sidings at Barking, Farringdon and near High Street Kensington (known as Triangle Sidings) stable trains overnight.