Communication via electronic systems such as telephones, faxes, e-mail, computers, etc., has enormously increased the volume and ease with which people and institutions can exchange messages and information. However, the associated technologies have also enabled the introduction of new sophisticated concepts and methods in interception and analysis for intelligence gatherers. One such method has been dubbed ECHELON and is used by which the United States and its partners in a worldwide intelligence alliance to intercept and analyse messages transmitted electronically from anywhere on Earth. The National Security Agency (NSA), based at Fort Mead in Maryland, is the US organisation most intimately involved in the operation of this covert surveillance system. This is the story of the methods developed and the institutions that adopt them and the debates and arguments that have accompanied their use from domestic surveillance to international commercial and political espionage.