After a period of use on bombing and reconnaissance, the type found its niche during the winter of 1940-41 as a night fighter defending Hitler's Reich. At first the three main crew members had no special equipment for night operations and relied on their eyes alone to find enemy aircraft in the dark. Ground-controlled interception began from mid 1941 and the 110 began to take its toll on RAF bombers and was soon an aircraft to be feared. Airborne radar was used experimentally during 1941, effective up to a maximum distance of 3.5 km/ 2.2 miles and capable of bringing the 110 to within 200m/655ft of a target.

The Bf 110 would be the backbone of the Nachtjagdgeschwader throughout the war. The first units undertook defence operations over Germany as early as the autumn of 1940. Opposition was light until 1942, when British heavy bombers started to appear.

One of the most notable actions of the Bf 110 occurred on the night of the 17/18 August 1943. Some Bf 110 units had been equipped with the experimental Schräge Musik system, an emplacement of two upward-firing cannon, which for its initial installations placed the twin-cannon fitment almost midway down the cockpit canopy behind the pilot, which could attack the blind spot of RAF Bomber Command's Lancaster and Halifax bombers, which lacked a ventral turret. Using this, NJG 5's Leutnant Peter Erhardt destroyed four bombers in 30 minutes. Despite excellent visibility, none of the RAF bombers had reported anything unusual that would indicate a new weapon or tactics in the German night fighter force. This ignorance was compounded by the tracerless ammunition used by the Bf 110s, as well as firing on the British bombers blind spots. Many RAF crews witnessed a sudden explosion of a friendly aircraft, but assumed, in some cases, it was very accurate flak. Few of the German fighters were seen, let alone fired on. Later on, as the specialist Bf 110G-4s were received by night fighter wings, the mid-cockpit mount was replaced by one at the extreme rear of the cabin.

In September 1943, Arthur Harris, convinced that a strategic bombing campaign against Germany's cities would force a German collapse, pressed for further mass attacks. While RAF Bomber Command destroyed Hannover's city centre and 86% of crews dropped their bombs within 5 km (3 mi) of the aiming point, losses were severe. The Ruhr Area was the prime target for British bombers in 1943, and German defences inflicted a considerable loss rate. The Bf 110 had a hand in the destruction of some 2,751 RAF bombers in 1943, along with German flak and other night fighters. Later, the RAF developed a radar countermeasure; Window, to blind German radar and introduced de Havilland Mosquitos to fly feints and divert the Bf 110s and other night fighter forces from their true target, which worked, initially. At this time, the Bf 110 remained the backbone of the night force, although it was now being reinforced by the Junkers Ju 88. In October 1943, General Josef Kammhuber reported the climbing attrition rate as "unacceptable", and urged Hermann Göring to stop committing the German night fighters to daylight operations. Many Nachtjagdgeschwader had taken part in costly daylight battles of attrition. From June–August, it had increased from around 2% to 9.8%. However the fortunes for the mostly Bf 110 equipped force turned during late August/September 1943. The night fighter arm claimed the destruction of 123 out of some 1,179 bombers over Hamburg on one night; a 7.2% loss rate.

During the Battle of Berlin, 1,128 bombers were lost in five months. RAF Bomber Command had "nearly burned out". These losses were primarily a result of fighter defences, at the heart of which was the Bf 110. The German defences had won a victory which prevented deep penetration raids for a time. But Luftwaffe losses were high; 15% of crews were killed in the first three months of 1944.

Most of the German night fighter aces flew the Bf 110 at some point during their combat careers, and the top night fighter ace of all time, Major Heinz-Wolfgang Schnaufer, flew it exclusively and claimed 121 victories in 164 combat missions.

Sources:
Gunston, Bill - The Encyclodepia of the Worlds Combat aircraft, 1976, Chartwell Books, Inc., New York
Brown, Eric, Captain - Wings of the Luftwaffe, 1979, Airlife Publishing Ltd., Shrewsbury
Gunston, Bill & Wood, Tony - Hitler's Luftwaffe, 1977, Salamander Books Ltd., London
Donald, David - The Complete Encyclopedia Of World Aircraft, 1997, Brown Packaging Books Ltd., London
Wikipedia - BF 110