The Secret Intelligence Service (SIS), commonly known as MI6 (Military Intelligence, Section 6) is the United Kingdom's external intelligence agency. Under the direction of the Joint Intelligence Committee (JIC), it works alongside the Security Service (MI5), Government Communications Headquarters (GCHQ) and the Defence Intelligence Staff (DIS). Within the civil service community the service is colloquially referred to as 'Box 850' which comes from its old post office box number.

Since 1995, the Secret Intelligence Service has had its headquarters at Vauxhall Cross on the South Bank of the Thames.