Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF, and given the official name Air Ministry Experimental Station Type 1 (AMES Type 1) in 1940, the radar units themselves were also known as Chain Home for most of their life. Chain Home was the first early warning radar network in the world, and the first military radar system to reach operational status. Its effect on the outcome of the war made it one of the most powerful weapons of what is today known as the "Wizard War".

In late 1934, the Tizard Committee asked radio expert Robert Watson-Watt to comment on the repeated claims of radio death rays and reports suggesting Germany had built some sort of radio weapon. His assistant, Arnold Wilkins, demonstrated that a death ray was impossible but suggested radio could be used for long-range detection. In February 1935, a demonstration was arranged by placing a receiver near a BBC shortwave transmitter and flying an aircraft around the area. An oscilloscope connected to the receiver showed a pattern from the aircraft's reflection. Funding quickly followed. Using commercial shortwave radio hardware, Watt's team built a prototype pulsed transmitter. On 17 June 1935, it successfully measured the angle and range of an aircraft that happened to be flying past. Basic development was completed by the end of the year, with detection ranges on the order of 100 miles (160 km). Through 1936 attention was focused on a production version, and early 1937 saw the addition of height finding.

The first five stations, covering the approaches to London, were installed by 1937 and began full-time operation in 1938. Operational tests that year, using early units, demonstrated the difficulties in relaying useful information to the pilots in fighter aircraft. This led to the formation of the first integrated ground-controlled interception network, the Dowding system, which collected and filtered this information into a single view of the airspace. Dozens of CH stations covering the majority of the eastern and southern coasts of the UK, along with a complete ground network with thousands of miles of private telephone lines, were ready by the time the war began in 1939. Chain Home proved decisive during the Battle of Britain in 1940. CH systems could detect enemy aircraft while they were forming over France, giving RAF commanders ample time to marshal their entire force directly in the path of the raid. This had the effect of multiplying the effectiveness of the RAF to the point that it was as if they had three times as many fighters, allowing them to defeat frequently larger German forces.

The Chain Home network was continually expanded, with over 40 stations operational by the war's end. CH was not able to detect aircraft at low altitude, and from 1939 was normally partnered with the Chain Home Low system, or AMES Type 2, which could detect aircraft flying at any altitude over 500 ft (150 m). Ports were covered by Chain Home Extra Low, which gave cover down to 50 ft (15 m) but at shorter ranges of approximately 30 miles (50 km). In 1942 the AMES Type 7 radar began to assume the job of tracking of targets once detected, and CH moved entirely to the early warning role. Late in the war, when the threat of Luftwaffe bombing had ended, the CH systems were used to detect V2 missile launches.

UK radar systems were wound down after the war, but the start of the Cold War led to efforts to establish a new network as rapidly as possible. In spite of being outdated, the Chain Home radars were pressed into service in the new ROTOR system until replaced by newer systems in the 1950s. Today only a few of the original sites remain intact in any fashion.