Espionage or espionage is the act of obtaining secret or confidential information without the permission of the information holder. Spies help agencies discover secret information. Under the service of government, company or independent organization, any individual or spy ring (spy cooperation group) can engage in espionage activities. This practice is secret because, by definition, it is unpopular, illegal in many cases and punishable by law. Espionage is a kind of "intelligence" collection method, which includes collecting information from public resources.

Espionage is usually part of an institutional effort by a government or business organization. However, the term is often associated with the state's espionage of potential or actual enemies for military purposes. Espionage involving companies is called industrial espionage.

One of the most effective ways to gather data and information about an enemy (or potential enemy) is to infiltrate the enemy's ranks. This is the job of a spy. Spies can return information about the size and strength of the enemy. They can also find dissidents in the enemy's forces and influence their defection. In times of crisis, spies steal technology and destroy the enemy in various ways. Counterintelligence is a way of defeating enemy spies and gathering intelligence. There are strict laws on espionage in almost all countries / regions, and the penalties for those arrested are often severe. But the benefits of espionage are often so great that most governments and many big companies are taking advantage of it.

The information collection technology used in secret human intelligence includes operation technology, asset raising and trade technology.