'Intruder' was the Allied overall designation for air operations against specific German activity, the most important of these being night-fighter missions to German night-flying bases for attacks on aircraft either taking off or landing, when they were particularly vulnerable (December 1940/May 1945).

The first such sorties were made on 21 December 1940 by Bristol Blenheim night-fighters of No. 23 Squadron of Air Marshal Sir William Sholto Douglas’s RAF Fighter Command, after responsibility had been transferred from Air Marshal Sir Richard Peirse’s RAF Bomber Command, which had flown 'security patrols' to harass Luftwaffe bombers participating in night raids on targets in the UK.