Espionage
History: Espionage has been recognized as of importance in military affairs since ancient times.
The oldest known classified document was a report made by a spy disguised as a diplomatic envoy in the court of King Hammurabi, who died in around 1750 BC. The ancient Egyptians had a developed secret service, and espionage is mentioned in the Iliad, the Bible, and the Amarna letters. Espionage was also prevalent in the Greco-Roman world, when spies employed illiterate subjects in civil services.
The thesis that espionage and intelligence has a central role in war as well as peace was first advanced in The Art of War and in the Arthashastra. In the Middle Ages European states excelled at what has later been termed counter-subversion when Catholic inquisitions were staged to annihilate heresy. Inquisitions were marked by centrally organised mass interrogations and detailed record keeping. Western espionage changed fundamentally during the Renaissance when Italian city-states installed resident ambassadors in capital cities to collect intelligence. Renaissance Venice became so obsessed with espionage that the Council of Ten, which was nominally responsible for security, did not even allow the doge to consult government archives freely. In 1481 the Council of Ten barred all Venetian government officials from making contact with ambassadors or foreigners. Those revealing official secrets could face the death penalty. Venice became obsessed with espionage because successful international trade demanded that the city-state could protect its trade secrets. Under Queen Elizabeth I of England (r. 1558–1603), Francis Walsingham (c. 1532–1590) was appointed foreign secretary and intelligence chief. The novelist and journalist Daniel Defoe (died 1731) not only spied for the British government, but also developed a theory of espionage foreshadowing modern police-state methods.
During the American Revolution, Nathan Hale and Benedict Arnold achieved their fame as spies, and there was considerable use of spies on both sides during the American Civil War. Though not a spy himself, George Washington was America's first spymaster, utilizing espionage tactics against the British.

In the 20th century, at the height of World War I, all great powers except the United States had elaborate civilian espionage systems and all national military establishments had intelligence units. In order to protect the country against foreign agents, the U.S. Congress passed the Espionage Act of 1917. Mata Hari, who obtained information for Germany by seducing French officials, was the most noted espionage agent of World War I. Prior to World War II, Germany and Imperial Japan established elaborate espionage nets. In 1942 the Office of Strategic Services was founded by Gen. William J. Donovan. However, the British system was the keystone of Allied intelligence. Numerous resistance groups such as the Austrian Maier-Messner Group, the French Resistance, the Witte Brigade, Milorg and the Polish Home Army worked against Nazi Germany and provided the Allied secret services with information that was very important for the war effort.
Since the end of World War II, the activity of espionage has enlarged, much of it growing out of the Cold War between the United States and the former USSR. The Russian Empire and its successor, the Soviet Union have had a long tradition of espionage ranging from the Okhrana to the KGB (Committee for State Security), which also acted as a secret police force. In the United States, the 1947 National Security Act created the Central Intelligence Agency (CIA) to coordinate intelligence and the National Security Agency for research into codes and electronic communication. In addition to these, the United States has 13 other intelligence gathering agencies; most of the U.S. expenditures for intelligence gathering are budgeted to various Defense Dept. agencies and their programs. Under the intelligence reorganization of 2004, the director of national intelligence is responsible for overseeing and coordinating the activities and budgets of the U.S. intelligence agencies.
In the Cold War, espionage cases included Alger Hiss, Whittaker Chambers and the Rosenberg Case. In 1952 the Communist Chinese captured two CIA agents and in 1960 Francis Gary Powers, flying a U-2 reconnaissance mission over the Soviet Union for the CIA, was shot down and captured. During the Cold War, many Soviet intelligence officials defected to the West, including Gen. Walter Krivitsky, Victor Kravchenko, Vladimir Petrov, Peter Deriabin, Pawel Monat and Oleg Penkovsky of the GRU. Among Western officials who defected to the Soviet Union are Guy Burgess and Donald D. Maclean of Great Britain in 1951, Otto John of West Germany in 1954, William H. Martin and Bernon F. Mitchell, U.S. cryptographers, in 1960, and Harold (Kim) Philby of Great Britain in 1962. U.S. acknowledgment of its U-2 flights and the exchange of Francis Gary Powers for Rudolf Abel in 1962 implied the legitimacy of some espionage as an arm of foreign policy.
China has a very cost-effective intelligence program that is especially effective in monitoring neighboring countries such as Mongolia, Russia and India. Smaller countries can also mount effective and focused espionage efforts. For instance, the Vietnamese communists had consistently superior intelligence during the Vietnam War. Some Islamic countries, including Libya, Iran and Syria, have highly developed operations as well. SAVAK, the secret police of the Pahlavi dynasty, was particularly feared by Iranian dissidents before the 1979 Iranian Revolution.

Modern day: Today, spy agencies target the illegal drug trade and terrorists as well as state actors.
Intelligence services value certain intelligence collection techniques over others. The former Soviet Union, for example, preferred human sources over research in open sources, while the United States has tended to emphasize technological methods such as SIGINT and IMINT. In the Soviet Union, both political (KGB) and military intelligence (GRU) officers were judged by the number of agents they recruited.

Targets of espionage: Espionage agents are usually trained experts in a targeted field so they can differentiate mundane information from targets of value to their own organizational development. Correct identification of the target at its execution is the sole purpose of the espionage operation.
Broad areas of espionage targeting expertise include:

Natural resources: strategic production identification and assessment (food, energy, materials). Agents are usually found among bureaucrats who administer these resources in their own countries
Popular sentiment towards domestic and foreign policies (popular, middle class, elites). Agents often recruited from field journalistic crews, exchange postgraduate students and sociology researchers
Strategic economic strengths (production, research, manufacture, infrastructure). Agents recruited from science and technology academia, commercial enterprises, and more rarely from among military technologists
Military capability intelligence (offensive, defensive, manoeuvre, naval, air, space). Agents are trained by military espionage education facilities and posted to an area of operation with covert identities to minimize prosecution
Counterintelligence operations targeting opponent's intelligence services themselves, such as breaching the confidentiality of communications and recruiting defectors or moles

Methods and terminology: Although the news media may speak of "spy satellites" and the like, espionage is not a synonym for all intelligence-gathering disciplines. It is a specific form of human source intelligence (HUMINT). Codebreaking (cryptanalysis or COMINT), aircraft or satellite photography (IMINT), and analysis of publicly available data sources (OSINT) are all intelligence gathering disciplines, but none of them is considered espionage. Many HUMINT activities, such as prisoner interrogation, reports from military reconnaissance patrols and from diplomats, etc., are not considered espionage. Espionage is the disclosure of sensitive information (classified) to people who are not cleared for that information or access to that sensitive information.
Unlike other forms of intelligence collection disciplines, espionage usually involves accessing the place where the desired information is stored or accessing the people who know the information and will divulge it through some kind of subterfuge. There are exceptions to physical meetings, such as the Oslo Report, or the insistence of Robert Hanssen in never meeting the people who bought his information.

The US defines espionage towards itself as "the act of obtaining, delivering, transmitting, communicating, or receiving information about the national defence with an intent, or reason to believe, that the information may be used to the injury of the United States or to the advantage of any foreign nation". Black's Law Dictionary (1990) defines espionage as: "... gathering, transmitting, or losing ... information related to the national defense". Espionage is a violation of United States law, 18 U.S.C. §§ 792–798 and Article 106a of the Uniform Code of Military Justice. The United States, like most nations, conducts espionage against other nations, under the control of the National Clandestine Service. Britain's espionage activities are controlled by the Secret Intelligence Service.

Technology and techniques: Source:

Organization: A spy is a person employed to seek out top secret information from a source. Within the United States Intelligence Community, "asset" is more common usage. A case officer or Special Agent, who may have diplomatic status (i.e., official cover or non-official cover), supports and directs the human collector. Cut-outs are couriers who do not know the agent or case officer but transfer messages. A safe house is a refuge for spies. Spies often seek to obtain secret information from another source.
In larger networks, the organization can be complex with many methods to avoid detection, including clandestine cell systems. Often the players have never met.