Espionage, spying, or intelligence gathering, as a subfield of the intelligence field, is the act of obtaining secret, confidential, or in some way valuable information. Such information is also referred to as intelligence. A person who commits espionage as part of a specific mission is called an espionage agent or spy. A person who commits espionage as a fully employed officer of a government is called an intelligence officer. Espionage may be conducted in a foreign country, domestically or remotely. Any individual or spy ring (a cooperating group of spies), in the service of a government, company, criminal organization, or independent operation, can commit espionage. The practice is clandestine, as it is by definition unwelcome. In some circumstances, it may be a legal tool of law enforcement and in others, it may be illegal and punishable by law.

Espionage is often part of an institutional effort by a government or commercial concern. The term is frequently associated with state spying on potential or actual enemies for military purposes. However, there are many types of espionage. Industrial espionage, for example, involves spying on civilians and their respective business or corporate interests.

One way spies gather data and information about a targeted military organization is by infiltrating its ranks. They can then return information such as the size and strength of the enemy forces. They can also find collaborators and dissidents within the organization and influence them to provide further information or defect. Spies can steal technology and sabotage the enemy in various ways. Counterespionage, also known as counterintelligence or offensive countertintelligence, is the practice of thwarting enemy espionage and intelligence gathering. Almost all sovereign states have laws concerning espionage and the penalties for being caught spying are often severe.

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. "The Art of War," identifies five types of spies that are essential for gathering intelligence and achieving victory: local spies (citizen informants within the enemy's territory), inward spies (recruited double agents within the enemy ranks), converted spies (recruited defectors converted to serve your side), doomed spies (expendable fabricators used to spread disinformation; acts as decoy for counter-intelligence), and surviving spies (spies that provide accurate intelligence after gathering information from the enemy).

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.