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.

Law.

Espionage against a nation is a crime under the legal code of many nations. In the United States, it is covered by the Espionage Act of 1917. The risks of espionage vary. A spy violating the host country's laws may be deported, imprisoned, or even executed. A spy violating its own country's laws can be imprisoned for espionage or and treason (which in the United States and some other jurisdictions can only occur if they take up arms or aids the enemy against their own country during wartime), or even executed, as the Rosenbergs were. For example, when Aldrich Ames handed a stack of dossiers of U.S. Central Intelligence Agency (CIA) agents in the Eastern Bloc to his KGB-officer "handler", the KGB "rolled up" several networks, and at least ten people were secretly shot. When Ames was arrested by the U.S. Federal Bureau of Investigation (FBI), he faced life in prison; his contact, who had diplomatic immunity, was declared persona non grata and taken to the airport. Ames' wife was threatened with life imprisonment if her husband did not cooperate; he did, and she was given a five-year sentence. Hugh Francis Redmond, a CIA officer in China, spent nineteen years in a Chinese prison for espionage—and died there—as he was operating without diplomatic cover and immunity.

In United States law, treason, espionage, and spying are separate crimes. Treason and espionage have graduated punishment levels.

The United States in World War I passed the Espionage Act of 1917. Over the years, many spies, such as the Soble spy ring, Robert Lee Johnson, the Rosenberg ring, Aldrich Hazen Ames, Robert Philip Hanssen, Jonathan Pollard, John Anthony Walker, James Hall III, and others have been prosecuted under this law.