How Spies Work

A man looking out from a window with binoculars.
Governments all over the world use spies to gather information. D-Keine / Getty Images

­­Real-wor­ld espionage rarely resembles the on-screen exploits of Hollywood secret agents. Still, spying is a useful and often dangerous way for governments to gather secret information from their enemies. The successes and failures of spies have shaped foreign policy, altered the course of wars and left a deep (though usually hidden) impression on world history.

­World leaders are faced with making important decisions every day, and information is the key to making the right decision. How many troops does your enemy have? How far are they in developing their secret weapons? Are they planning to negotiate a trade deal with another country? Are some of their generals planning a military coup?


While some of this information (known as intelligence) may be readily available, most countries keep information that could be used against them secret. Of course, this secret information is often the most valuable. To gain access to secret information, governments use espionage, a blend of subterfuge, deception, technology and data analysis. Espionage can also be used to counteract the spying efforts of the enemy, mainly be supplying them with false information.

Creating Spies

Spies are recruited in a number of ways. Some join the intelligence agencies of their home countries, receive training and move on to jobs within the agency. If their background and training fits a certain profile, they may be sent abroad to take on a cover identity (more on this in the next section).

The best field agents are those with access to high-ranking­ officials or secret information in other countries. Spy agencies employ recruiters, people who target citizens of other countries who are likely to turn against their homeland and become spies. These defectors are invaluable spies, since they already have a cover and can provide information almost immediately. There are several factors that can cause someone to defect and become a spy:

  • Ideological disagreement with their home country. During the Cold War, the KGB (the Russian abbreviation for Committee for State Security, the Soviet Union's intelligence and secret police agency) had success recruiting agents in the United States and Britain who were known to support communism or belong to communist organizations.
  • Money. Many spies have turned over crucial, deadly information for nothing more than cash.
  • Desire to be "important". Recruiters look for people in menial positions who have access to important information. Certain psychological factors can drive some people to become spies because it makes them feel powerful.
  • Blackmail. Recruiters who hold evidence of behavior that their target would not want made public, such as an extramarital affair, can threaten to release the evidence if the target doesn't agree to become a spy. Threats of physical harm to the target or her family members also work well.

­On rare occasions, no recruiting work is needed at all. Someone who wants to provide information walks in to an embassy or consulate and offers to become a spy. These walk-ins may be viewed with distrust as potential sources of misinformation from the enemy, but they can also become valuable spies.

Once a recruiter has recruited someone willing to gather information, the new spy will be put in contact with a controller. The controller will offer some training in spying methods and issue instructions for obtaining and transmitting information. The spy will usually have contact with no one else, never learning the names of any other spies or officials. This is known as compartmentalization. Each spy works within his own compartment, so if he is captured and interrogated, he can't reveal vital information or the identities of other spies.


Spy Methods

This shot of Mys Shmidta Air Field in the former USSR is one of the first images taken by the U.S. spy satellite CORONA in 1960.
Photo courtesy National Reconnaissance Office

­Methods of acquiring information are as varied as the information as itself. The most important element of a long-­term spying operation is the use of a cover and the creation of a legend. A cover is a secret identity, and a legend is the background story and documents that support the cover.

For example, a British agent whose cover identity is a Russian accountant would need to speak Russian and know a great deal about Russian financial laws. To make the cover seem more realistic, the legend must be very thorough. The agent will have a fake life history that he must memorize. Where did he go to school? Does he have a diploma to prove it? Where was he born? Who is his ex-wife? What are his hobbies? If the legend states that the agent enjoys fishing, he'd better have some fishing gear in his house. The failure or success of spies can hinge on such seemingly minor details.


Once the spy establishes a cover, he might spend years doing his job and establishing trust. Eventually, the spy will try to gain promotions or transfers to a position with access to vital information, or befriend someone with such access.

It is possible for a spy to memorize information and pass it on to his controller. However, it is much more reliable to photocopy papers and maps, often transferring the data to a tiny slip of microfilm or a microdot. Stealing original documents could blow the spy's cover, so a wide assortment of miniature cameras hidden in innocuous objects are used. For more details on these devices, check out How Spy Gadgets Work.

There are numerous technological ways for countries to spy on each other without ever sending an actual spy to collect information. Satellites equipped with cameras have been tracking the positions of military units since the 1960s. At first, the satellite would drop a bucket with the film inside over the ocean. In 1970, digital film technology was first developed, allowing the satellites to transmit the photographic data by radio. Today's spy satellites can take photographs with a high enough resolution to read the headline on a newspaper.

It was more difficult to get that kind of accuracy in the '60s and '70s -- spy planes such as the U-2 had to fly directly over enemy territory, exposing the pilot to the risk of being shot down, and the spying nation to the risk of international embarrassment.

Other forms of Tech Int, or technological intelligence, include super-sensitive microphones, phone wire taps, seismic equipment to detect nuclear testing and underwater sensors to find enemy submarines. Spies also scan, record and analyze enemy radio frequencies and cell-phone traffic.



Passing Information

A hidden letter that Aldrich Ames left at a dead drop for his KGB contact
Photo courtesy U.S. Department of Justice

­When secret information is passed to the spy's controllers, it must be hidden so that the enemy doesn't suspect anything. This could ruin the spy's cover, or lead the enemy to deliberately supply misinformation. Until the early 20th century, spies resorted to invisible inks to hide messages between the lines or on the bac­k of non-suspicious correspondence. Sugar solutions or lemon juice are invisible until heated. Other chemicals don't appear until the paper is painted with a specific reagent.

One time-tested method for relaying information is the dead drop. A dead drop is a secret hiding place somewhere in public. It could be behind a loose brick in a wall at the city park, or in a plant at a certain street corner. When a spy has a message to send, he goes about his business, perhaps picking up some dry cleaning or seeing a movie. He passes by the dead drop and deposits the message casually, without arousing suspicion. The spy then has to leave a signal to let his handlers know there is a message to be retrieved. A chalk mark on a lamppost, a certain color of sheet on a clothes lines or even a cryptic message in the classified section of a newspaper are all possible signals. A spy may use several dead drops so he isn't noticed repeatedly visiting the same loose brick.


Spy controllers can use one-way communication t­o issue instructions to spies. The mysterious numbers stations in operation around the world are almost certainly used for this purpose. A numbers station is a government operated radio station broadcasting intermittently on the short-band frequencies. A certain song or announcement will mark the beginning and end of each broadcast, which consists solely of a voice, possibly altered electronically, reciting a long series of numbers. The numbers are coded messages deciphered by the intended recipient using a virtually unbreakable cipher known as a one-time pad.

A great deal of espionage revolves around secret codes. Information transmitted between spies and controllers is usually coded, and a large proportion of government and military communications are encoded, particularly during wars. Many spy missions have the sole purpose of acquiring the keys needed to solve these codes, or obtaining the devices used to encode and decode messages.


Data Analysis

The acquisition and transmission of secret information is meaningless if the information isn't properly analyzed and acted upon. Russian leader Joseph Stalin was provided with information from several agents that Germany was going to break the German-Russian alliance and attack Russia during World War II, but he refused to believe it. Russian forces were not properly aligned or prepared when the German attack came.

Data analysts take information from numerous sources, not just spies, and develop an overall picture of enemy strategies and policies. This information is then written into briefings for political leaders. While information from a single source may be untrustworthy, additional sources can be used to corroborate the data. For example, U.S. code breakers had partially cracked the Japanese Purple code during World War II, and they were fairly certain that Japan was planning an attack at Midway Island. They weren't completely sure if they were reading Japan's code word for the island (AF) correctly, however, so they had troops positioned at Midway to issue a radio alert saying they were running short on fresh water. Shortly, Japanese communications were intercepted that reported that AF was low on fresh water, confirming the target of the coming attack.



­Spies spend as much time feeding false information to their enemies as they do gathering information. This keeps them guessing, forces them to miscalculate military capabilities and commit forces to the wrong area. A steady strea­m of misinformation can even damage the real information the enemy has, because they will begin to doubt the authenticity of their own intelligence gathering activities.

One method of spreading misinformation is the double agent. Imagine that a U.S. scientist is recruited by the Russians to supply American military technology. The United States becomes aware that this scientist is spying for the Russians. Instead of arresting him, they allow him to continue feeding information to the Russians. However, they make sure that the blueprints, technical readouts and other data he has access to are altered. The Russians are now getting technical information that is useless. They might spend millions of dollars on research into flawed technology. Thus, the scientist is an unwitting double agent.


Alternately, the United States could confront the scientist and threaten him with a long prison sentence (or even the death sentence, the penalty for treason). To avoid this, he agrees to intentionally turn double agent. Not only does he knowingly supply the Russians with false information, but he works to gain information from his Russian controller. He might provide the United States with the names of other Russian spies, or intelligence on the level of Russian scientific research.

There is always the potential for this same scientist/spy to turn triple agent. That is, he informs the Russians that the Americans caught him. Now, the Russians know to disregard the technical information that he provides, and in turn, they supply misinformation back to the Americans. If it seems confusing, imagine trying to keep it all straight when a mistake could cost you your life. Some agents have even gone beyond triple agent, playing the two sides against one another and creating such a tangled web that historians have no idea whose side the spy was really on.

Operation Fortitude was one of the grandest and most successful misinformation campaigns ever conducted. The goal of Fortitude was to fool the Germans into withholding their strongest military units or putting them in the wrong place when the Allies invaded Normandy in 1944. Wooden and cardboard airplanes, fake fuel depots and even dummy troops were massed in southern England to make the Germans think the attack would come from there, rather than at Normandy in the north. A completely fictional U.S. Army group was created: FUSAG (First U.S. Army Group), which even had General George Patton leading it. False radio traffic supplemented the deception. The most important element, however, was the misinformation provided to the Germans by double agents. Information supplied by a double agent code-named Garbo convinced Hitler that the attack would come from the south.

To keep up the pretense and delay the arrival of German reinforcements in Normandy as long as possible, the day of the invasion even featured a fake landing force with loudspeakers playing the sounds of a giant fleet moving across the English Channel, with radar-reflecting balloons and metal strips dropped by planes creating the radar signature of a large invasion. Once the attack at Normandy was underway, Garbo told his German handlers that it was just a feint meant to draw German troops away from the "real" attack to the south.

For lots more information on spies, espionage and related topics, check out the links on the next page.


Lots More Information

Relat­ed HowStuffWorks Articles

More Great Links


  • Carlisle, Rodney, Ph.D. "The Complete Idiot's Guide to Spies and Espionage." Alpha, April 1, 2003. ISBN 0028644182.
  • Keeley, Jennifer. "American War Library - The Cold War: Espionage." Lucent Books, 1st edition, October 21, 2002. ISBN 1590182103.
  • Owen, David. "Spies: The Undercover World of Secrets, Gadgets and Lies." Firefly Books, September 2004. ISBN 1552977951.
  • "Spy." The American Heritage Dictionary of the English Language, 4th edition.
  • "Spy." Merriam-Webster Online Dictionary.
  • Volkman, Ernest. "Spies: The Secret Agents Who Changed the Course of History." Wiley, New edition, June 17, 1997. ISBN 0471193615.
  • Yancey, Diane. "Spies (History Makers)." Lucent Books, November 2001. ISBN 1560069589.