From Citizendium, the Citizens' Compendium
- Clandestine human-source intelligence and covert action : Intelligence and military special operations functions that either should be completely secret (i.e., clandestine: the existence of which is not known outside the relevant government circles), or simply cannot be linked to the sponsor (i.e., covert: it is known that sabotage is taking place, but its sponsor is unknown).
- Counterintelligence : Countermeasures to foreign intelligence organizations collecting intelligence against one's own side.
- Clandestine human-source intelligence operational techniques : "Tradecraft" of espionage and activities supporting it, such as secret communication, document forgery, etc.
- Direction finding : Finding the location of an electromagnetic emitter, either by the crossed bearings from directional antennas from various known points, or by time of arrival or time difference of arrival of the signal at those points
- Federal Bureau of Investigation : The principal U.S. Federal police agency, part of the U.S. Department of Justice and the United States intelligence community, who has arrest authority, and is the primary authority for a variety of domestic crimes, civilian counterespionage within the United States, and organized crime
- Organs of State Security : A generic term for Soviet intelligence and internal security organizations, from the Cheka to the KGB; traditionally a third of the power balance among the Communist Party of the Soviet Union and the Red Army; term continues in Russian usage with the FSB
- Committee for State Security : Usually known as the KGB, one of Organs of State Security of the Soviet Union, with extensive responsibilities in intelligence (information gathering), border security, counterintelligence and suppression of dissent; split up in the Russian Federation with counterintelligence in the FSB and foreign intelligence in the SVR
- Security Service : British domestic counterintelligence service, without police powers
- Reich Main Security Administration : Add brief definition or description
- Abwehr : German military counterintelligence and external intelligence in World War II, eventually absorbed into the Nazi party security apparatus other than purely military support
- Shin Bet : Israeli domestic intelligence, roughly equivalent to the British Security Service (MI5)