Government Communications Headquarters (GCHQ) is an intelligence and security organisation responsible for providing signals intelligence (SIGINT) and information assurance (IA) to the government and armed forces of the United Kingdom. Primarily based at The Doughnut in the suburbs of Cheltenham, GCHQ is the responsibility of the country's Secretary of State for Foreign and Commonwealth Affairs (Foreign Secretary), 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 responsible for breaking the German Enigma codes. There are two main components of GCHQ, the Composite Signals Organisation (CSO), which is responsible for gathering information, and the National Cyber Security Centre (NCSC), 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.

In 2013, GCHQ received considerable media attention when the former National Security Agency contractor Edward Snowden revealed that the agency was in the process of collecting all online and telephone data in the UK via the Tempora programme. Snowden's revelations began a spate of ongoing disclosures of global surveillance. The Guardian newspaper was forced to destroy computer hard drives with the files Snowden had given them because of the threats of a lawsuit under the Official Secrets Act. In June 2014, The Register reported that the information the government sought to suppress by destroying the hard drives related to the location of a "beyond top secret" GCHQ internet monitoring base in Seeb, Oman, and the close involvement of BT and Cable & Wireless in intercepting internet communications.

GCHQ is led by the director of GCHQ, Anne Keast-Butler, and a corporate board, made up of executive and non-executive directors. Reporting to the Corporate Board are:

During the First World War, the British Army and Royal Navy had separate signals intelligence agencies, MI1b and NID25 (initially known as Room 40) respectively.

In 1919, the Cabinet's Secret Service Committee, chaired by Lord Curzon, recommended that a peacetime codebreaking agency should be created, a task which was given to the Director of Naval Intelligence, Hugh Sinclair. Sinclair merged staff from NID25 and MI1b into the new organisation, which initially consisted of around 25–30 officers and a similar number of clerical staff. It was titled the "Government Code and Cypher School" (GC&CS), a cover-name which was chosen by Victor Forbes of the Foreign Office. Alastair Denniston, who had been a member of NID25, was appointed as its operational head. It was initially under the control of the Admiralty and located in Watergate House, Adelphi, London. Its public function was "to advise as to the security of codes and cyphers used by all Government departments and to assist in their provision", but also had a secret directive to "study the methods of cypher communications used by foreign powers". GC&CS officially formed on 1 November 1919, and produced its first decrypt prior to that date, on 19 October.

Before the Second World War, GC&CS was a relatively small department. By 1922, the main focus of GC&CS was on diplomatic traffic, with "no service traffic ever worth circulating" and so, at the initiative of Lord Curzon, it was transferred from the Admiralty to the Foreign Office. GC&CS came under the supervision of Hugh Sinclair, who by 1923 was both the Chief of SIS and Director of GC&CS. In 1925, both organisations were co-located on different floors of Broadway Buildings, opposite St. James's Park. Messages decrypted by GC&CS were distributed in blue-jacketed files that became known as "BJs". In the 1920s, GC&CS was successfully reading Soviet Union diplomatic cyphers. However, in May 1927, during a row over clandestine Soviet support for the General Strike and the distribution of subversive propaganda, Prime Minister Stanley Baldwin made details from the decrypts public.

During the Second World War, GC&CS was based largely at Bletchley Park, in present-day Milton Keynes, working on understanding the German Enigma machine and Lorenz ciphers. In 1940, GC&CS was working on the diplomatic codes and ciphers of 26 countries, tackling over 150 diplomatic cryptosystems. Senior staff included Alastair Denniston, Oliver Strachey, Dilly Knox, John Tiltman, Edward Travis, Ernst Fetterlein, Josh Cooper, Donald Michie, Alan Turing, Gordon Welchman, Joan Clarke, Max Newman, William Tutte, I. J. (Jack) Good, Peter Calvocoressi and Hugh Foss. The 1943 British–US Communication Intelligence Agreement, BRUSA, connected the signal intercept networks of the GC&CS and the US National Security Agency (NSA). Equipment used to break enemy codes included the Colossus computer. Colossus consisted of ten networked computers.