Naomi Grimley asks if the modern media can keep national security secrets.
Twice a year, over tea and biscuits at the Ministry of Defence, senior media editors meet senior civil servants to talk about what should be kept secret in the military, intelligence and counter-terrorism worlds.
Originally known as the D-Notice Committee, it's been in existence for nearly a century.
It started out dominated by newspaper proprietors, now though even Google is a member.
In D for Discretion Naomi Grimley asks where does the public's right to know end and the state secret start? And can the media even be trusted to keep such secrets in the internet age?
In the early days the remit of the D-Notice Committee was wide.
Newspapers, for example, weren't supposed to make any mention of Rasputin and his relationship with "the highest personage in Russia".
Nowadays, though, the system is supposed to be used only in the most serious cases when national security may be at stake.
The "Defence Advisory Notice System" - as it is now called - is supposed to be entirely voluntary.
In reality, though, it's very rare for any of the mainstream media organisations to ignore the committee's requests.
But how does this work in the age of Wikileaks and citizen journalism? This programme looks at the challenges to the system posed by social media websites.
What happens if members of the public try to reveal government secrets on Twitter - in a similar way to this year's row about super-injunctions? And how do newspapers like The Guardian square their Wikileaks collaborations with their own editorial guidelines on national security issues?
Produced by Alicia Trujillo.