Counterintelligence (counter-intelligence) or counterespionage (counter-espionage) is any activity aimed at protecting an agency's intelligence program from an opposition's intelligence service. It includes gathering information and conducting activities to prevent espionage, sabotage, assassinations or other intelligence activities conducted by, for, or on behalf of foreign powers, organizations or persons.

Many countries will have multiple organizations focusing on a different aspect of counterintelligence, such as domestic, international, and counter-terrorism. Some states will formalize it as part of the police structure, such as the United States' Federal Bureau of Investigation (FBI). Others will establish independent bodies, such as the United Kingdom's MI5, others have both intelligence and counterintelligence grouped under the same agency, like the Canadian Security Intelligence Service (CSIS).

Modern tactics of espionage and dedicated government intelligence agencies developed over the course of the late-19th century. A key background to this development was the Great Game – the strategic rivalry and conflict between the British Empire and the Russian Empire throughout Central Asia between 1830 and 1895. To counter Russian ambitions in the region and the potential threat it posed to the British position in India, the Indian Civil Service built up a system of surveillance, intelligence and counterintelligence. The existence of this shadowy conflict was popularized in Rudyard Kipling's famous spy book, Kim (1901), where he portrayed the Great Game (a phrase Kipling popularized) as an espionage and intelligence conflict that "never ceases, day or night".

The establishment of dedicated intelligence and counterintelligence organizations had much to do with the colonial rivalries between the major European powers and to the accelerating development of military technology. As espionage became more widely used, it became imperative to expand the role of existing police and internal security forces into a role of detecting and countering foreign spies. The Evidenzbureau (founded in the Austrian Empire in 1850) had the role from the late-19th century of countering the actions of the Pan-Slavist movement operating out of Serbia.

After the fallout from the Dreyfus affair of 1894–1906 in France, responsibility for French military counter-espionage passed in 1899 to the Sûreté générale—an agency originally responsible for order enforcement and public safety—and overseen by the Ministry of the Interior.

The Okhrana initially formed in 1880 to combat political terrorism and left-wing revolutionary activity throughout the Russian Empire, was also tasked with countering enemy espionage. Its main concern was the activities of revolutionaries, who often worked and plotted subversive actions from abroad. It set up a branch in Paris, run by Pyotr Rachkovsky, to monitor their activities. The agency used many methods to achieve its goals, including covert operations, undercover agents, and "perlustration"—the interception and reading of private correspondence. The Okhrana became notorious for its use of agents provocateurs, who often succeeded in penetrating the activities of revolutionary groups – including the Bolsheviks.

Integrated counterintelligence agencies run directly by governments were also established. The British government founded the Secret Service Bureau in 1909 as the first independent and interdepartmental agency fully in control over all government counterintelligence activities.

Due to intense lobbying from William Melville and after he obtained German mobilization plans and proof of their financial support to the Boers, the British government authorized the formation of a new intelligence section in the War Office, MO3 (subsequently redesignated MO5) headed by Melville, in 1903. Working under-cover from a flat in London, Melville ran both counterintelligence and foreign intelligence operations, capitalizing on the knowledge and foreign contacts he had accumulated during his years running Special Branch.