Typhoon pilots are on standby 24 hours a day to defend UK airspace.
Squadrons are based at RAF Coningsby in Lincolnshire, covering the south of the country, and RAF Lossiemouth in Moray, to cover the north.
The Quick Reaction Alert (QRA) crews can take off within minutes to intercept aircraft which have caused concern.
This can be because they are Russian military aircraft, or civilian planes which have stopped communicating with air traffic control, are not following their flight plan or have sent an emergency signal.
On some occasions pilots are given permission to fly at supersonic speeds - which may results in a sonic boom - to reach the aircraft as soon as possible.
Ministry of Defence figures show that QRA were launched on 12 days last year - eight in response to Russian aircraft and four to investigate other planes.
In January 2015 Typhoons were scrambled when two Russian Tu-95 Bear planes were flying close to UK airspace.
The Foreign Office said the incident was part of an "increasing pattern of out-of-area operations by Russian aircraft".
The Russian ambassador was summoned to account for what had happened.
Typhoons are under the direction of controllers at RAF Boulmer in Northumberland and RAF Scampton in Lincolnshire, although any decision to shoot down an aircraft could only be taken at a high political level.