The collapse of the prosecution of Katharine Gun under the Official Secrets Act has put the Government Communications Headquarters (GCHQ) firmly in the spotlight.
GCHQ Cheltenham has 4,500 staff
The 29-year-old was a translator there before her arrest last year for leaking an e-mail to the Observer newspaper, but little is known about one of the UK's chief intelligence and security organisations.
GCHQ is part of the civil service and reports to the foreign secretary.
It works closely with the UK's other intelligence agencies, MI5 and MI6, and mainly serves the Ministry of Defence, Foreign Office and law enforcement authorities.
Set up in 1946 out of the wartime code-breakers based in Bletchley Park, it is based in Cheltenham, Gloucestershire, in a brand new building.
Its two functions are called "signals intelligence" and "information assurance".
The signals intelligence work is the provision of information to support the government in areas of national security, military operations and law enforcement.
This requires monitoring, intercepting and decrypting information from those who pose a threat to the UK.
Much of this intelligence is used in the struggle against terrorism and the prevention of serious crime.
Information assurance helps to keep confidential data secret by protecting the government's communication and IT systems from hackers and other threats.
GCHQ also has this responsibility to protect the IT systems of bodies responsible for the UK's critical national infrastructure, such as power, water and communications providers.
Many of its 4,500 staff are skilled mathematicians and linguists.
GCHQ is accountable to Parliament and senior members of the judiciary and its activities are governed by the Intelligence Services Act 1994, amended most recently by the Anti-Terrorism, Crime and Security Act 2001, and the Regulation of Investigatory Powers Act 2000.
For instance, the gathering of intelligence, particularly the interception of information, is restricted by these Acts.
Interception can only be carried out for the sake of national security, safeguarding economic wellbeing and the prevention and detection of serious crime.
The impressive £337m Cheltenham building that became the home of GCHQ last year is nicknamed "the doughnut".
It was built on a circular design, big enough to house the old Wembley Stadium, including a courtyard large enough to take the Royal Albert Hall.
Fourteen staff lost their jobs in 1986 from the previous Cheltenham base after a decision by Margaret Thatcher's government to ban union membership.
They were compensated a total of £500,000 in 2000.