Counter-intelligence or counter-espionage is the act of seeking and indentifying espionage activities.
Major nations have organizations which perform 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.
Counter-intelligence agencies
See also
