David Cameron described the Queen's reign as a "golden thread" running through the generations.
Since 1952 she has worked with 12 prime ministers and 6 Archbishops of Canterbury during a time of unprecedented social and political change.
Historian David Starkey says her success has been in staying out of politics and remaining true to her sense of duty.
Her Majesty's cousin, Margaret Rhodes, 89, says that Elizabeth has reigned the way her father, George VI, wanted her to.
ITV News Correspondent Paul Davies takes a look back at the longest reign the British Isles has ever seen: