The National Security Agency/Central Security Service (NSA/CSS) is the United States government's cryptologic intelligence agency, administered under the U.S. Department of Defense. Created on November 4, 1952, it is responsible for the collection and analysis of foreign communications and foreign signals intelligence, which involves a significant amount of cryptanalysis. It is also responsible for protecting U.S. government communications and information systems from similar agencies elsewhere, which involves a significant amount of cryptography.

The NSA is directed by a lieutenant general or vice admiral. The NSA is a key component of the U.S. Intelligence Community, which is headed by the Director of National Intelligence. The Central Security Service is a co-located agency created to coordinate intelligence activities and co-operation between the NSA and U.S. military cryptanalysis agencies. Contrary to popular impression, the NSA's work is limited to communications intelligence and not field or human intelligence activities. By law, the NSA's intelligence gathering is limited to foreign communications; surveillance within the U.S. is prohibited to the NSA (except for communication with foreign agents) and is conducted by the Federal Bureau of Investigation.