ECHELON is a name used to describe a highly secretive world-wide signals intelligence and analysis network said to be run by the UKUSA Community (comprised of intelligence agencies of five English-speaking nations), that has been reported by a number of sources including, in 2001, a committee of the European Parliament. Its existence was first revealed by Duncan Campbell in a 1988 article, "Someone's listening," published in the New Statesman. According to some sources ECHELON can capture radio and satellite communications, telephone calls, faxes, e-mails and other data streams nearly anywhere in the world and includes computer automated analysis and sorting of intercepts. The EP committee, however, concluded that "the analysis carried out in the report has revealed that the technical capabilities of the system are probably not nearly as extensive as some sections of the media had assumed".

Reportedly created to monitor the military and diplomatic communications of the Soviet Union and its East Bloc allies during the Cold War in the early sixties, today ECHELON is believed to search also for hints of terrorist plots, drug-dealers' plans, and political and diplomatic intelligence. But some critics, including the European Union committee that commissioned the EU report, claim the system is being used also for large-scale commercial theft and invasion of privacy.

While details of methods and capabilities are highly sensitive and protected by special laws (e.g. 18 USC 798), gathering signals intelligence (SIGINT) is an acknowledged mission of the U.S. National Security Agency. As of August 2006, their web site had a FAQ page on the topic, which states:

"NSA/CSS’s Signal Intelligence mission is to intercept and analyze foreign adversaries' communications signals, many of which are protected by codes and other complex countermeasures. We collect, process, and disseminate intelligence reports on foreign intelligence targets in response to intelligence requirements set at the highest levels of government. ... Foreign intelligence means information relating to the capabilities, intentions, and activities of foreign powers, organizations or persons"

In 2001, the EP report (p. 19) recommended that citizens of member states routinely use cryptography in their communications to protect their privacy. In the UK, the government introduced the Regulation of Investigatory Powers Act which gives authorities the power to demand that citizens hand over their encryption keys, without a judge-approved warrant. In April 2004, the European Union decided to spend 11 million euros developing secure communication based on quantum cryptography — the SECOQC project — a system that would theoretically be unbreakable by ECHELON or any other espionage system. European governments have been leery of ECHELON since a December 3, 1995 story in the Baltimore Sun claiming that aerospace company Airbus lost a $6 billion contract with Saudi Arabia in 1994 after the NSA reported that Airbus officials had been bribing Saudi officials to secure the contract.