MI5, properly known as the Security Service, is the UK's security intelligence agency. Although the "MI" stands for "Military Intelligence", MI5 is a civilian agency without formal executive powers. It is principally concerned with countering threats to domestic security - in contrast to MI6, which is concerned with external security. Historically, MI5's focus has been counter-espionage directed against foreign subversion: today, it is focused on counter-terrorism work and fighting organised and serious crime.

MI5 today works primarily in liaison with other agencies, particularly the Special Branch of police forces. Its officers do not have any powers of arrest or detention.

MI6, properly known as the Secret Intelligence Service (SIS), is the UK's external intelligence agency - that is, it is responsible for protecting and promoting British interests and security overseas. One of the methods by which it does this is espionage. The "MI" in MI6's informal title also stands for "Military Intelligence".

The employer of the world's most famous fictional spy, James Bond, MI6 is a highly secretive organisation, whose work remains shrouded in mystery - much more so than its domestic security counterpart MI5.

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