Method of obtaining information about enemy or potential enemy by stealing their secret data. Espionage is strictly forbidden in most countries and heavily punished as work against the state security, but many countries still perform such actions against others, nevertheless.

The spy is one of the oldest occupation in history. The importance of accurate data about enemy was pointed in Sun Tzu's The Art of War. Having knowledge about its weak and strong points allows to prepare better to fight and thus destroy enemy forces with smaller losses.

The Intelligence is state force that is responsible for espionage, but also gathering information that are not secret, but can be useful for military purposes.

The espionage can also mean acts of sabotage on enemy territory, performing s against enemy leaders, destroying communication lines, halting weapon factories, secretly spreading propaganda to lower morale of enemy troops, or preparing bases for strike-forces like commandos, Rangers or the Spetsnaz. A secret agent can also be used for just reconnaissance purposes.

Spies can recruit other spies from enemy's citizens by using such methods like bribery and blackmails.

Spies operate both during peace and wartime. They are recruited both from men and women. In modern times techniques like breaking into enemy's computer networks by some sort of hackers are also performed.

The police or military force intended to fight enemy spies and keep own secrets is called counterintelligence.

Spies that are caught are usually treated as traitors and are punished very strongly. In wartime typical punishment is death. In previous eras, to frighten other enemy spies, also tortures were committed. However, sometimes, it is more useful to simulate that the spy has still not been discovered and use him to send false information to the enemy (so-called double-agent). Sometimes spies are not punished, but exchanged by own spies that were caught on enemy territory.

In modern times also industrial espionage occurs. It is when company attempts to steal its competitor secrets to improve own market position.

The popular culture view on spies is much over-rated. Fictional characters like James Bond have almost nothing related to real espionage world.

Some famous spies:

Notable institutions (former and still acting) responsible for spying:

Questions tagged with should be related to history of this method of obtaining information. Also questions about famous espionage actions are accepted. You can mark with this tag questions about some persons who were spies, secret agents or supervisors of espionage forces.