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.
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 viewed from 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 the death penalty. In many legal systems, there may be reduced legal protections during criminal investigations and trials for those who are accused of threatening national security.
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; especially World War I, World War II in Europe and Cold War Europe.
In the typical case, an intelligence 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 the Russian Federation, and Mossad of Israel. Their staff consists of intelligence officers, who should not be confused with spies, secret agents or assets, who are recruited (usually from within a target organization) to provide secret information. There are two main categories of spies: A mole is recruited by an intelligence agency, and then makes a career in the target organization to get secret information, whereas a defector is an official who decides to provide information for the enemy (in some cases fleeing their homeland). A double 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 of diplomatic missions. Diplomatic immunity limits the host country's enforcement methods; a common reaction is to expel a suspected intelligence officer as a persona non grata. Intelligence officers and agents recruited outside the diplomatic system are called illegalists or non-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.
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 need cryptanalysis 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 and space flight.
A security 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. A secret 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 the Soviet 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 in Hunt 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.
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.
Spy Museum Berlin, 52.50897°, 13.37931°. 2018-02-12
Berlin Friedrichstraße station, 52.520278°, 13.386944°. During the Cold War war, this railway station was a major crossing point between East and West Germany. The station is said to have been used by several spies to cross the border and is mentioned in several spy novels. It was also "tear palace" where tearful goodbyes between members of families separated by the wall took place after visits. 2018-02-12
Former KGB Prison Potsdam (Gedenk- und Begegnungsstätte Ehemaliges KGB-Gefängnis Potsdam), 52.41646°, 13.06434°. Memorial and meeting place at the former KGB prison. From August 1945 it was occupied by soviet forces. It has been reconstructed as a prison for the counterintelligence. Today it's been left standing to remind people of the depressing reality of dictatorships.
Glienicke Brücke, 52.4134°, 13.0901°. Once a border crossing during the Cold War, this bridge is best known as the Bridge of Spies, and was used as an exchange point for captured spies between the United States and the Soviet Union.
Stasi Museum, 52.514444°, 13.4875°. This museum describes the procedures applied by the East German secret police. 2018-08-25
Gedenkstätte Hohenschönhausen (Stasi Prison), 52.5417°, 13.5011°. This is the former prison used by East Germany's infamous "Stasi" secret police. While overt torture was only used in the early years of this prison, the Stasi developed ever more sophisticated means to get information or confessions out of inmates. Tours are compulsory. Some of the tours are done by former inmates. 2018-08-25
The Five 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 since World War II, when the British at Bletchley Park broke nearly all the German and Italian ciphers and the US broke many Japanese ciphers. They expanded their co-operation during the Cold 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 an investigation 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.
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.
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 as ECHELON or the Great 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. See Internet_access#Security_concerns and the EFF page on Surveillance self-defense for other methods.
Related Wikipedia article: Category:Espionage museums