Counter-intelligence is the activity of preventing the enemy from obtaining secret information, such as careful classification and control of sensitive information and spreading disinformation.

Counter-espionage is spying directed against an enemy's spy system, such as by recruiting agents in foreign intelligence organs.

Most governments and militaries have organizations which specifically fulfill this role. Methods include surveillance of suspects and their communications, undercover agents, monitoring the behavior of legally accredited 'diplomatic personnel' (some of whom are sometimes actually spies or spy handlers), and similar means.

When spying is discovered, the agencies usually have arrest power, but it is often more productive to keep a careful eye on them to see what they know, where they go, and who they talk to. Furthermore, disinformation can be used to fool the spies and their sponsors, or make them cease their activities if they learn their information has become unreliable and/or their secrecy has been compromised. Intelligence and counter-intelligence activities occur not only between governments but between industries as well as criminal and terrorist groups.