Real-life Spooks speak about life as a spy
2 April 2012 Last updated at 19:15 BST
They inhabit a shadowy world with much about their work and lives classified "Top Secret".
They are the men and women of MI5 and MI6.
In a new BBC2 documentary series, serving spies talk publicly for the first time about what it takes to operate deep undercover.
The BBC's Peter Taylor reports.
''Modern Spies" is broadcast on BBC2 at 9pm tonight.