ECHELON, originally a secret government code name, is a surveillance program (signals intelligence / SIGINT collection and analysis network) operated on behalf of the five signatory nations to the UKUSA Security Agreement—Australia, Canada, New Zealand, the United Kingdom and the United States, also known as the Five Eyes.

The ECHELON program was created in the late 1960s to monitor the military and diplomatic communications of the Soviet Union and its Eastern Bloc allies during the Cold War, and was formally established in 1971.

By the end of the 20th century, the system referred to as "ECHELON" had allegedly evolved beyond its military and diplomatic origins, to also become "…a global system for the interception of private and commercial communications.

Britain's "The Guardian" newspaper summarized the capabilities of the ECHELON system as follows:

A global network of electronic spy stations that can eavesdrop on telephones, faxes and computers. It can even track bank accounts. This information is stored in Echelon computers, which can keep millions of records on individuals.

Officially, however, Echelon doesn't exist.