Counterintelligence (CI) refers to efforts made by intelligence organizations to prevent hostile or enemy intelligence organizations from successfully gathering and collecting intelligence against them. National intelligence programs, and, by extension, the overall defenses of nations, are vulnerable to attack. It is the role of intelligence cycle security to protect the process embodied in the intelligence cycle, and that which it defends. A number of disciplines go into protecting the intelligence cycle. One of the challenges is there is a wide range of potential threats, so threat assessment, if complete, is a complex task.

Many governments organize counterintelligence agencies separate and distinct from their intelligence collection services for specialized purposes. In most countries the counterintelligence mission is spread over multiple organizations, though one usually predominates. There is usually a domestic counterintelligence service, perhaps part of a larger law enforcement organization such as the FBI in the United States. Great Britain has the separate Security Service, also known as MI5, which does not have direct police powers but works closely with law enforcement called the Special Branch that can carry out arrests, do searches with a warrant, etc. Russia's major domestic security organization is the FSB, which principally came from the Second Chief Directorate of the USSR KGB. Canada separates the functions of general defensive counterintelligence (contre-ingérence), security intelligence (the intelligence preparation necessary to conduct offensive counterintelligence), law enforcement intelligence, and offensive counterintelligence.

Military organizations have their own counterintelligence forces, capable of conducting protective operations both at home and when deployed abroad. Depending on the country, there can be various mixtures of civilian and military in foreign operations. For example, while offensive counterintelligence is a mission of the US CIA's National Clandestine Service, defensive counterintelligence is a mission of the U.S. Diplomatic Security Service (DSS), Department of State, who work on protective security for personnel and information processed abroad at US Embassies and Consulates.

The term counter-espionage is really specific to countering HUMINT, but, since virtually all offensive counterintelligence involves exploiting human sources, the term "offensive counterintelligence" is used here to avoid some ambiguous phrasing.

Among the differences found in American English and British English, some confusion is created by the use of or absence of a hyphen in the word counterintelligence, with the former often omitting the hyphen and the latter incorporating it. Both spellings are correct, and likely to appear in this article and others.