The Security Service, commonly known as MI5 (Military Intelligence, Section 5), is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service (SIS; also known as MI6) focused on foreign threats, Government Communications Headquarters (GCHQ) and Defence Intelligence (DI).
All come under the direction of the Joint Intelligence Committee (JIC). The service has a statutory basis in the Security Service Act 1989 and the Intelligence Services Act 1994. Its remit includes the protection of British parliamentary democracy and economic interests, counter-terrorism and counter-espionage within the UK. Although mainly concerned with internal security, it does have an overseas role in support of its mission.
Within the civil service community the service is colloquially known as Box 500 (after its official wartime address of PO Box 500; its current address is PO Box 3255, London SW1P 1AE).
The service has had a national headquarters at Thames House on Millbank in London since 1995, drawing together personnel from a number of locations into a single HQ facility. Thames House was, until March 2013, shared with the Northern Ireland Office (NIO) and is also home to the Joint Terrorism Analysis Centre, a subordinate organisation to the Security Service. The service has offices across the United Kingdom including an HQ in Northern Ireland.