The Queen's reign has seen 12 American presidents in office and she has met all of them except one.
Lyndon B Johnson, who took over in 1963 following John F Kennedy's assassination, never crossed paths with the monarch.
The Queen today met US President Barack Obama for lunch at Windsor Castle - the day after her 90th birthday.
A collection of archive photographs shows Elizabeth II's encounters through the decades with the leaders of one of the UK's closest allies.
In 1951, Princess Elizabeth travelled to Washington and met President Harry S Truman, who was still in the Oval Office when she acceded to the throne.
During her first state visit to America in 1957, now a Queen, she met Dwight D Eisenhower. They met again in Canada in 1959 and travelled on the Royal Yacht Britannia together.
The Queen was said to have had an affectionate relationship with Mr Eisenhower who was even invited to stay at the Queen's Scottish hideaway Balmoral Castle.
JF Kennedy and his wife Jackie dined with the Queen and the Duke of Edinburgh at Buckingham Palace in June 1961, as did Richard Nixon in February 1969.
The Queen also met Mr Nixon at Chequers in October 1970 and was photographed with him outside the prime minister's official country residence.
In 1976, during the US bicentennial, she met Gerald Ford in the US, and in 1977 she was pictured in a billowing, wide-sleeved yellow gown alongside Jimmy Carter, in a black tie and tuxedo, when he came to London.
Ronald and Nancy Reagan stayed at Windsor Castle in 1982. The monarch and the former film star shared a bond through their love of horses and rode side by side in Windsor Great Park.
She also met Mr Reagan during an official visit to America's West Coast in 1983.
George Bush Snr had lunch with the Queen at Buckingham Palace in 1989 and she visited him in the US in 1991 - the trip that led to "Podiumgate" - when all that could be seen of the Queen above the lectern when she was delivering an address on the White House lawn was her hat.
Bill Clinton and his wife Hillary - who is now herself campaigning to be president - met the Queen several times, including in December 2000 when, along with daughter Chelsea, they dropped in on the Palace, as did George W Bush in July 2001.
Two years later in November 2003, Mr Bush paid an official state visit and the Queen gave him a wide smile as they shook hands at her London residence.
When the Queen encountered Mr Bush again in Washington DC in 2007, the president winked at her after making a gaffe.
At a welcome ceremony on the south lawn of the White House, he mixed up his dates in a speech - almost ageing the Queen by 200 years.
Mr Bush - in 2008 - and Mr Reagan were both received by the Queen at Windsor Castle.
Her relationship with the outgoing Barack Obama has been a warm and friendly one.
They hit it off after a reception ahead of the G20 summit in 2009. A successful state visit to the UK followed in 2011 and the Obamas have forged a strong bond with Queen and her wider family - particularly Prince Harry and the Duke and Duchess of Cambridge.