The Security Service comes under the authority of the Home Secretary within the Cabinet. The service is headed by a Director General at the grade of a Permanent Secretary of the British Civil Service who is directly supported by an internal security organisation, secretariat, legal advisory branch and information services branch. The Deputy Director General is responsible for the operational activity of the service, being responsible for four branches; international counter-terrorism, National Security Advice Centre (counter proliferation and counter espionage), Irish and domestic counter-terrorism and technical and surveillance operations.
The service is directed by the Joint Intelligence Committee for intelligence operational priorities. It liaises with SIS, GCHQ, DIS, and a number of other bodies within the British government and industrial base. It is overseen by the Intelligence and Security Committee of Members of Parliament, who are directly appointed by the Prime Minister, by the Interception of Communications Commissioner, and by the Intelligence Services Commissioner. Judicial oversight of the service's conduct is exercised by the Investigatory Powers Tribunal.
Operations of the service are required to be proportionate and compliant with British legislation including the Regulation of Investigatory Powers Act 2000, the Data Protection Act 1998, and various other items of legislation. Information held by the service is exempt from disclosure under section 23 of the Freedom of Information Act 2000.
All employees of the service are bound by the Official Secrets Act. In certain circumstances, employees can be authorised to carry out activity which would otherwise be criminal within the UK.
The current Director General is Andrew Parker, who succeeded Jonathan Evans on 22 April 2013.
The service marked its centenary in 2009 by publishing an official history titled The Defence of the Realm: The Authorized History of MI5 and written by Christopher Andrew, Professor of Modern and Contemporary History at Cambridge University.