There is a long history of close cooperation between the United States and the United Kingdom intelligence services; see Clandestine HUMINT and Covert Action for World War II and subsequent relationships. There are permanent liaison officers of each country in major intelligence agencies of the other, such as CIA and Secret Intelligence Service ("MI6") (which is the British counterpart of the CIA), FBI and the Security Service (MI5), and National Security Agency (NSA) and Government Communications Headquarters (GCHQ).