While most of their political and personal conversations are kept strictly confidential, memoirs and historic interactions provide an insight into their relationships.
So as Theresa May becomes Britain's next Prime Minister -- and Her Majesty's 13th -- take a look back at those who have served her.
WINSTON CHURCHILL 1951-1955
The Queen was said to be in awe of her first prime minister, Winston Churchill. Once when asked which PM she enjoyed meeting with most, she replied: "Winston of course, because it's always such fun."
ANTHONY EDEN 1955-1957
Her Majesty found her second prime minister to be a sympathetic listener and their relationship was one of constitutional propriety. The largest political event to occur during Eden's time was the Suez crisis. During this time, he believed it was of supreme importance to keep the Queen informed, so he shared all of the Suez papers with her -- the first time she had ever been shown secret government documents.
HAROLD MACMILLAN 1957-1963
The Queen originally found Macmillan difficult to deal with, but they eventually warmed to each other. Her Majesty relied on Macmillan for his wise counsel -- both while in office and after his retirement in 1963.
ALEC DOUGLAS-HOME 1963-1964