In military aviation, an intruder is a fighter aircraft, often a night fighter, the crew of which are tasked with penetrating deep into enemy airspace to disrupt enemy air operations. To achieve this they: attack fighters, airfields, radar and other infrastructure; stage diversionary attacks; and escort bombers. Intruders often loiter in the vicinity of enemy airbases to attack aircraft as they take off or land.
During World War II, the British Royal Air Force deployed the Hawker Hurricane Mk IIc as a makeshift intruder in various theatres. One of the first aircraft modified as a specialised intruder was the Douglas Havoc I.
From late 1943, Bristol Beaufighters and de Havilland Mosquito intruders patrolled over occupied Europe, using Serrate radar detectors to hunt German nightfighters.