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.
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
The Queen was well acquainted with Douglas-Home as he had been a childhood friend of the Queen Mother. So Her Majesty worked hard to re-establish her informal relationship with him. Over the year he was in office, Douglas-Home helped the monarch name several royal horses.
HAROLD WILSON 1964-1970, 1974-1976
Wilson, who came from a lower-middle-class background, became the Queen's first Labour Party prime minister. Wilson often broke away from meeting traditions, and enjoyed helping with the washing-up after barbecues at Balmoral -- one of the Queen's residences. The Queen, however, warmed to Wilson's informal presence and even invited him to stay for drinks after their first meeting, which was not commonplace.
EDWARD HEATH 1970-1974