Aircraft interception radar, or AI radar for short, is a historical British term for radar systems used to equip aircraft with the means to find and track other flying aircraft. These radars are used primarily by Royal Air Force (RAF) and Fleet Air Arm night fighters and interceptors for locating and tracking other aircraft, although most AI radars could also be used in a number of secondary roles as well. The term was sometimes used generically for similar radars used in other countries, notably the US. AI radar stands in contrast with ASV radar, whose goal is to detect ships and other sea-surface vessels, rather than aircraft; both AI and ASV are often designed for airborne use.

The term was first used circa 1936, when a group at the Bawdsey Manor research center began considering how to fit a radar system into an aircraft. This work led to the AI Mk. IV radar, the first production air-to-air radar system. Mk. IV entered service in July 1940 and reached widespread availability on the Bristol Beaufighter by early 1941. The Mk. IV helped end the Blitz, the Luftwaffe's night bombing campaign of late 1940 and early 1941.

Starting with the AI Mk. VII, AI moved to microwave frequencies using the cavity magnetron, greatly improving performance while reducing size and weight. This gave the UK an enormous lead over their counterparts in the Luftwaffe, an advantage that was to exist for the remainder of World War II. By the end of the war, over a dozen AI models had been experimented with, and at least five units widely used in service. This included several US-built models, especially for the Fleet Air Arm.

The AI naming convention was used in the post-war era as well, but these generally dropped the "Mk." when written in short form and used numbers instead of Roman numerals. A good example is the AI.24 radar of the Tornado ADV. These radars were often given common names as well, and generally better known by these; the AI.24 is almost universally referred to as "Foxhunter". Other widely used post-war examples include the AI.18 used on the de Havilland Sea Vixen, and the AI.23 Airpass on the English Electric Lightning. This article will use Mk. or AI. depending on which is most commonly used in available references.

In order to provide the maximum possible warning time of an incoming raid, the RAF's Chain Home (CH) radar stations had been positioned as far forward as possible, right on the coastline. These systems could only see targets in front of them, over the English Channel. Tracking over land fell to the Royal Observer Corps (ROC) using visual means. In testing it was found that the two different reporting systems provided information that varied enough to make tracking targets confusing and error prone, and the sheer volume of information could be overwhelming.

Hugh Dowding addressed this through the creation of what is today known as the Dowding system, networking together the radars and observation centres by telephone to a central station. Here, in the Fighter Command's "filter room" at RAF Bentley Priory, operators would plot the map coordinates sent to them on a single large map, which allowed them to correlate multiple reports of the same target into a single track. Telephone operators, or "tellers", would then forward this information to group headquarters who would re-create the map, and then from group to the sector HQs who would give instructions to the fighter pilots.

Due to delays in the flow of information between the various centres, and inherent inaccuracies in the reports coming from multiple sources, this system was accurate to perhaps 5 miles (8.0 km). Within 5 miles the fighters would normally be able to spot their targets visually and complete the interception on their own. Interception rates over 80% was common, and on several occasions the system succeeded in getting every fighter launched into position for an attack.

While the Dowding system proved invaluable inputs during daylight attacks, it was essentially useless against night raids. Once the enemy aircraft passed the coastline they could not be seen by the radars, and the ROC could not see at night except under ideal conditions with bright moonlight, no cloud cover, and considerable luck. Even when tracks could be developed, the difficulty of spotting a target from the cockpit of an aircraft while flying it at night proved to be equally difficult. Henry Tizard wrote a memo on the topic in 1936, indicating that the Germans would likely begin a night campaign if the daylight campaign went as poorly as he believed it would due to Chain Home.