This is the official website of the Security Service (MI5). The Security Service is the UK's security intelligence agency. Its purpose is to protect national security from threats such as terrorism, espionage and the proliferation of weapons of mass destruction, to safeguard the economic well-being of the UK against foreign threats, and to support the law enforcement agencies in preventing and detecting serious crime. This website outlines the nature and work of the Service. The content is an updated version of the booklet MI5: The Security Service (3rd edition), published by the Stationery Office in 1998.