Ever since there have been organised states, there have been agents working behind the curtains, as well as things that regimes have sought to protect from prying eyes and ears.
The revealed activities of both these agents and sites are fascinating to some travellers, even if the themes exposed, events and specific methods (aka tradecraft) involved can seem brutal, controversial or in some instances downright surreal.
 | Note: Beware that spying is among the most closely guarded of government activities. There are museums, and sites like that in the photo above which can generally be safely viewedfrom outside, though even wandering about the area or taking photos might bring unwelcome attention from security personnel. Intruding on a spy site without permission is illegal anywhere, almost certain to get you arrested and subjected to ungentle questioning. Beyond that, any penalty is possible, in some places including thedeath penalty. In many legal systems, those who are accused of threatening national security have less legal protection than ordinary criminals. |
| “ | Hence it is that which none in the whole army are more intimate relations to be maintained than with spies. None should be more liberally rewarded. In no other business should greater secrecy be preserved. | ” |
—Sun Tzu,The Art of War | ||
While the history of espionage goes back to ancient times, it is usually the spy stories of the 20th century that have created the public impression of the business; especiallyWorld War I,World War II in Europe and theCold War.
In the typical case, anintelligence agency is a government organ which collects information about foreign governments and other target organizations. Some of them are CIA of the United States, SIS (or MI6) of the United Kingdom, SVR of Russia, and Mossad of Israel. Their staff consists ofintelligence officers, who should not be confused withspies,secret agents orassets, who are recruited (usually from within a target organization) to provide secret information. There are two main categories of spies: Amole is recruited by an intelligence agency, and then makes a career in the target organization to get secret information, whereas adefector is an official who decides to provide information for the enemy (in some cases fleeing their homeland). Adouble agent is an agent who pretends to work for the intelligence organization that recruited them, but is loyal to the target organization. In some cases, people who are "outed" as double agents then proceed to become "triple agents" and so on, although this is more common in fiction than in reality.
Intelligence officers regularly work under cover ofdiplomatic missions. Diplomatic immunity limits the host country's enforcement methods; a common reaction is to expel a suspected intelligence officer as apersona non grata. Intelligence officers and agents recruited outside the diplomatic system are calledillegalists ornon-official cover. Little is known about the extent of those.
In a few cases, an intelligence agency can carry out assassinations and sabotage operations against enemies, either using their own officers or through an agent. Intelligence agencies also often work with local political dissidents in foreign countries to overthrow governments deemed unfriendly to the interests of the agency's country; the overthrow of Mohammad Mosaddegh inIran in 1953, and the failedTibetan Uprising in 1959 are prominent of examples of local revolutions assisted by the CIA.
There are several operation methods.Signals intelligence (SIGINT) is a highly specialized intelligence process which intercepts radio and telecommunication signals, usually in a separate agency such as NSA in the United States, or GCHQ in the United Kingdom. Secret messages are usually encrypted (cryptography is the art of creating ciphers), intelligence agencies needcryptanalysis to decipher them. From World War II to the 21st century, cryptology and signals intelligence have been supported by government "big science" projects to develop electronics, computer technology, aviation andspace flight.
Asecurity agency fights crime and espionage (counterespionage), and protects strategic targets such as leaders or buildings; in the U.S.A., both the FBI and the Secret Service have this function, while the corresponding agency in the UK is the Security Service (or MI5), Russia's equivalent is the FSB, and Israel's equivalent is known as Shin Bet. Asecret police is a government security agency directed against internal opposition or other individuals and groups a more or less authoritarian government has decided to harass, arrest or worse. Examples include the Gestapo of Nazi Germany and the Kenpeitai of Imperial Japan. Some government agencies have had multiple roles, such as the KGB of theSoviet Union, which was both an intelligence service and a secret police force.
Despite the glorious and mysterious image surrounding espionage, most intel is actually gathered in rather "boring" ways. One example is to measure the security preparedness of the US by the number of cars in the CIA parking lot. Another example is Tom Clancy who had to answer some uncomfortable questions about "revealing military secrets" for his description of submarine engines inHunt for Red October - turns out he could point to books available in public libraries as the source of all his information, combined with a bit of educated guessing. The most famous spies are ironically those who failed their missions and got caught.
Intelligence agencies are often alleged to have ties toorganized crime syndicates. For instance, both the CIA and the KGB were alleged to have been involved in drug trafficking to finance their operations during the Cold War.
In the 21st century,cyberwarfare is increasingly becoming an important part of conflicts between nations, in many cases even more so than actual military action. The countries generally considered to have the best cyberwarfare capabilities are the United States, Russia, Israel, the United Kingdom and China.
Most facilities for intelligence and national security are very restricted to visitors. Intrusive photography is prohibited, in many cases. The most interesting places to visit are usually museums, and decommissioned buildings.
(updated Feb 2018) (updated Mar 2017) 
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(updated Feb 2018) (updated Feb 2018)(updated Feb 2018)(updated Jul 2020)(updated Feb 2018)TheFive Eyes, an alliance of the major English-speaking democracies— the United States, United Kingdom, Canada, Australia and New Zealand— have been co-operating on signals intelligence sinceWorld War II, when the British atBletchley Park broke nearly all the German and Italian ciphers and the US broke many Japanese ciphers. They expanded their co-operation during theCold War and continue it today. The best-known of their projects was codenamed ECHELON. This is widely considered the most powerful spying alliance in the world.
The governments involved still consider everything about these programs top secret, but quite a lot about them has become public. The main sources are aninvestigation by the European Parliament early in this century and leaks by former NSA contractor Edward Snowden in 2013.
They have a worldwide network of intercept stations and can collect nearly all satellite communications plus much else. We list some here; see Wikipedia for a much longer list.
There are also stations in several friendly countries, at least Germany, Japan, Brazil and India. Controversially, they were also revealed to have stations located within their respective embassies.
None of these sites are open to the public, there is not much to see from outside— just big antennas or domes which cover antennas— and photographing them may be actively discouraged.
| “ | Dance like no-one is watching. Encrypt like everyone is. | ” |
Most people are at almost no risk of being seriously spied on, though if you visit a country whose government is hostile to yours and/or is highly authoritarian, the risk may be larger. You may also be at risk if you have valuable commercial or research data. If professionals— government, corporate or criminal— specifically target you and you do not have professionals on your side, then your chances of successful defense are somewhere between slim and nil.
However, there are also less specific threats such as advertisers tracking web activity or government programs that hoover up data en masse, such asECHELON or theGreat Firewall of China. It is possible to defend reasonably well against these.
One simple defense is to use a "burner" phone or laptop; set it up before travel with as little valuable data as possible and wipe all data when you return. SeeInternet_access#Security_concerns and the EFF page onSurveillance self-defense for other methods.
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