The UK government’s eavesdropping agency, it carries out electronic surveillance to combat terrorism, cyber threats and serious crime.
It’s one of three UK intelligence agencies, the others being MI5, the domestic security service, and MI6, the foreign intelligence service.
GCHQ’s collaboration with the United States dates back to the Second World War.
Its operations are shrouded in secrecy and it rarely makes public statements.
It has its headquarters in the English spa town of Cheltenham and says it employs more than 6,000 people.