The "thirty year rule" is the popular name given to a law in the United Kingdom, the Republic of Ireland and Australia that states that the yearly cabinet papers of a government will be released publicly thirty years after they were created.

Usually this occurs on 1 January of a particular year. The only papers not released (or if they are contained in other papers 'blacked out') are those which are deemed likely to cause "damage to the country's image, national security or foreign relations" if they were to be released.

A good example of this was when the British cabinet papers for 1973 were released - the papers covering September 11 were barred from release as that was the day of the coup by Augusto Pinochet against Chilean President Salvador Allende.

See also: Sunshine law, Freedom of Information Act