The Government Communications Headquarters (GCHQ) is a British intelligence and security organisation responsible for providing signals intelligence (SIGINT) and information assurance to the British government and armed forces. Based in "The Doughnut", in the suburbs of Cheltenham, it operates under the formal direction of the Joint Intelligence Committee (JIC) alongside the Security Service (MI5), the Secret Intelligence Service (MI6) and Defence Intelligence (DI). GCHQ is the responsibility of the UK Secretary of State for Foreign and Commonwealth Affairs, but it is not a part of the Foreign Office and its Director ranks as a Permanent Secretary.

GCHQ was originally established after the First World War as the Government Code and Cypher School (GC&CS) and was known under that name until 1946. During the Second World War it was located at Bletchley Park, where it was famed for its role in the breaking of the German Enigma codes. Currently there are two main components of the GCHQ, the Composite Signals Organisation (CSO), which is responsible for gathering information, and the CESG, which is responsible for securing the UK's own communications. The Joint Technical Language Service (JTLS) is a small department and cross-government resource responsible for mainly technical language support and translation and interpreting services across government departments. It is co-located with GCHQ for administrative purposes.