Counterintelligence or counter-intelligence (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 Federal Bureau of Investigation in the United States. The United Kingdom 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. The Russian Federation's major domestic security organization is the FSB, which principally came from the Second Chief Directorate of the USSR KGB and Third Chief Directorate of the KGB USSR. 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.