The District line is a London Underground line running from Upminster in the east and Edgware Road in the west to Earl's Court in west London, where it splits into multiple branches. One branch runs to Wimbledon in south-west London and a short branch, with a limited service, only runs for one stop to Kensington (Olympia).The main route continues west from Earl's Court to Turnham Green after which it divides again into two western branches, to Richmond and Ealing Broadway.

Printed in green on the Tube map, the line serves 60 stations (2 stations for Paddington) over 40 miles (64 km). It is the only Underground line to use a bridge to traverse the River Thames, crossing on both the Wimbledon and Richmond branches. The track and stations between Barking and Aldgate East are shared with the Hammersmith & City line, and between Tower Hill and Gloucester Road and on the Edgware Road branch they are shared with the Circle line. Some of the stations between South Kensington and Ealing Common are shared with the Piccadilly line. Unlike London's deep-level lines, the railway tunnels are just below the surface.

The S Stock trains are maintained at Ealing Common Depot[a] and Upminster Depot.[b] Ealing Common Depot was built by the District Railway when it was electrified in the early 1900s. Upminster depot was built 1956–1958 when the District line tracks were segregated. Trains may also be stabled in the sidings east of Barking alongside Hammersmith & City Line trains.