The Security Service, often known as MI5, is the UK's national security intelligence agency.
Since 1909 we have been responsible for countering covertly organised threats to national security. We also provide security advice to a range of other organisations, helping them reduce their vulnerability to the threats.
Our staff, headed by Director General Jonathan Evans, are largely based in our headquarters at Thames House in London. We also have eight regional offices around the UK plus a Northern Ireland headquarters. The Service is organised into seven branches, each with specific areas of responsibility, who work to counter a range of threats including terrorism, espionage and the proliferation of weapons of mass destruction.
The Service has played a vital but necessarily secret role over the past century in countering the activities of terrorists and spies. Since the late 1990s we have been releasing historical files to the UK's National Archives, providing the public with a unique insight into the work of the Service and a behind-the-scenes look at famous spy cases of the 20th century. You can find out more about the evolution and historical activities of the Security Service in our history pages.