The Avro Lancaster was a British four-engined Second World War heavy bomber made initially by Avro for the Royal Air Force (RAF). It first saw active service in 1942, and together with the Handley Page Halifax it was one of the main heavy bombers of the RAF, the RCAF and squadrons from other Commonwealth and European countries serving within RAF Bomber Command. The "Lanc", as it was affectionately known, became the most famous and most successful of the Second World War night bombers, "delivering 608,612 tons of bombs in 156,000 sorties." Although the Lancaster was primarily a night bomber, it excelled in many other roles including daylight precision bombing, and gained worldwide renown as the "Dam Buster" used in the 1943 Operation Chastise raids on Germany's Ruhr Valley dams.

The first RAF squadron to convert to the Lancaster was No. 44 Squadron RAF in early 1942.

Lancasters flew 156,000 sorties and dropped 608,612 long tons (618,378 tonnes) of bombs between 1942 and 1945. Lancs took part in the devastating round-the-clock raids on Hamburg during Air Marshall Harris' "Operation Gomorrah" in July 1943. Just 35 Lancasters completed more than 100 successful operations each, and 3,249 were lost in action. The most successful survivor completed 139 operations, and was scrapped in 1947.

A famous Lancaster bombing raid was the 1943 mission, codenamed Operation Chastise, to destroy the dams of the Ruhr Valley. The operation was carried out by 617 Squadron in modified Mk IIIs carrying special drum shaped bouncing bombs designed by Barnes Wallis. The story of the operation was later made into a film, The Dam Busters. Also famous was a series of Lancaster attacks using Tallboy bombs against the German battleship Tirpitz, which first disabled and later sank the ship.

Lancasters from Bomber Command were to have formed the main strength of Tiger Force, the Commonwealth bomber contingent scheduled to take part in Operation Downfall, the codename for the planned invasion of Japan in late 1945, from bases on Okinawa.

RAF Lancasters dropped food into the Holland region of the occupied Netherlands, with the acquiescence of the occupying German forces, to feed people who were in danger of starvation. The mission 'Operation Manna' named after the food Manna which miraculously appeared for the Israelites in the Book of Exodus, the aircraft involved were from 1, 3 and 8 Groups, and consisted of 145 Mosquitoes and 3,156 Lancasters, flying between them a total of 3,298 sorties. The first of the two RAF Lancasters chosen for the test flight was nicknamed "Bad Penny" from the old expression: "a bad penny always turns up." This bomber, with a crew of seven men (five Canadians including pilot Robert Upcott of Windsor, Ontario), took off in bad weather on the morning of 29 April 1945 without a ceasefire agreement from the German forces, and successfully dropped her cargo.

A development of the Lancaster was the Avro Lincoln bomber, initially known as the Lancaster IV and Lancaster V. These two marks became the Lincoln B1 and B2 respectively. There was also a civilian airliner based on the Lancaster, the Lancastrian. Other developments were the York, a square-bodied transport and, via the Lincoln, the Shackleton which continued in airborne early warning service up to 1992.

In 1946, four Lancasters were converted by Avro at Bracebridge Heath, Lincolnshire as freighters for use by British South American Airways, but proved to be uneconomical and were withdrawn after a year in service.

Four Lancaster IIIs were converted by Flight Refuelling Limited as two pairs of tanker and receiver aircraft for development of in-flight refuelling. In 1947, one aircraft was flown non-stop 3,459 mi (5,567 km) from London to Bermuda. Later the two tanker aircraft were joined by another converted Lancaster and were used in the Berlin Airlift, achieving 757 tanker sorties.

Fifty-nine Lancaster B.I and B.VII were overhauled by Avro at Woodford and Langar and delivered to the Aeronavale (France) during 1952/53. These were flown until the mid-1960s by four squadrons in France and New Caledonia in the maritime reconnaissance and search-and-rescue roles.

During its Argentinian service, Lancasters saw limited use in military coups, owing to the small number there.