Espionage, spying, or intelligence gathering is the act of obtaining secret or confidential information (intelligence). A person who commits espionage is called an espionage agent or spy. 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. However, the term tends to be associated with state spying on potential or actual enemies for military purposes. Spying involving corporations is known as industrial espionage.

One way to gather data and information about a targeted organization is by infiltrating its ranks. Spies can then return information such as the size and strength of enemy forces. They can also find dissidents within the organization and influence them to provide further information or to defect. In times of crisis, spies steal technology and sabotage the enemy in various ways. Counterintelligence is the practice of thwarting enemy espionage and intelligence-gathering. Almost all sovereign states have strict laws concerning espionage, including those who practise espionage in other countries, and the penalties for being caught are often severe.