Intelligence service, institution of a national government that gathers information; particularly about clandestine activities of other countries or enemies, for the purpose of protecting national security. Such functions were once performed chiefly by foreign ambassadors. Under Elizabeth I, England was among the first Western countries to set up an elaborate intelligence service, privately financed by Sir Francis Walsingham. Joseph Fouché established an equally vital network for Napoleonic France. Britain's intelligence service reached its highest point during World War II, with the brilliant amateurs of MI-5.
In the United States, military espionage first assumed great importance during the Civil War, in which it was employed extensively by both sides. During World War I the U.S. intelligence service was organized largely by the Office of Strategic Services (OSS). It was given its present form in 1947 with the creation of the Central Intelligence Agency (CIA).
The CIA, which came under investigation and some censure in the late 1970s, coordinates the intelligence functions of all government departments and agencies. All the armed services have their own intelligence branches.