Despite his current illness, the Duke of Edinburgh has never been far from the Queen's side. He's the man she has called her "constant strength and guide".
The couple celebrated their 64th wedding anniversary on 20 November 2011.
Their marriage has spanned seven decades, four children, eight grandchildren, Accession and Coronation and The Queen's 60-year reign.
Here are a few quick facts about the 90-year-old Duke:
The Duke of Edinburgh was born Prince Philip of Greece and Denmark in Corfu on 10 June 1921.
He is the oldest living great, great-grandchild of Queen Victoria
The Duke of Edinburgh, is Patron or President of some 800 organisations
Prince Philip was the first President of World Wildlife Fund - UK
Marriage and family
The engagement of Lieutenant Philip Mountbatten to Princess Elizabeth was announced in July 1947.
The couple married in Westminster Abbey on 20 November 1947.
The Queen and Prince Philip had two children before (Prince Charles and Princess Anne) and two after (Prince Andrew and Prince Edward) The Queen succeeded to the throne.
Before Princess Elizabeth’s accession to the throne on 6 February 1952, Prince Philip had a distinguished career in the Royal Navy.
Once his wife became Queen, the Duke sacrificed his career as a Commander in the Royal Navy to fulfill his duties as a royal consort.
In addition to his royal duties, the Duke was a keen sportsman, taking part in charity cricket matches in the 1950s and playing polo until 1971.
The Prince took up polo while serving in Malta and played regularly until 1971.
He also represented Great Britain at several European and World four-in-hand Driving Championships.
During the Queen's reign, in addition to his own duties and charitable work, the Duke of Edinburgh is nearly always by his wife's side, accompanying her on numerous public engagements and foreign visits, and being a source of support when she faced challenging times.
In a speech to mark their Golden Wedding anniversary in 1997, the Duke said of their marriage:
"The main lesson we've learned is that tolerance is the one central ingredient of any happy marriage. It might not be so important when things are going well but it is absolutely vital when things get difficult."
The following day, in speech at London's Guildhall in 1997, the Queen paid a warm tribute to her husband saying:
“He has, quite simply, been my strength and stay all these years, and I, and his whole family, and this and many other countries, owe him a debt greater than he would ever claim or we shall ever know.”
In March 2012, in a speech to celebrate her Diamond Jubilee, the Queen once again paid tribute to her husband, who she said was "a constant strength and guide."
On April 7 2011, the photographer Thomas Struth took a picture of the Queen and Duke of Edinburgh to mark the Diamond Jubilee.
It shows the Queen and Prince Philip seated together in the Green Drawing Room at Windsor Castle.
Struth told the BBC:
"I wanted to leave them both in their royal environment, and of course not try to disguise who they are, but also show them as an elderly couple who are together."
In January 2012, to mark his grandmother's reign, Prince Harry paid tribute to his grandfather.
He told the broadcaster Andrew Marr :
"Regardless of whether my grandfather seems to be doing his own thing, sort of wandering off like a fish down the river, the fact that he’s there – personally, I don’t think that she could do it without him, especially when they’re both at this age."
The Duke of Cambridge also recently praised Prince Philip in an interview for the ITV documentary Elizabeth: Queen, Wife, Mother.
Prince William told Alan Titchmarsh the Queen appreciated her husband’s well-known sense of humour, even if it sometimes gets him into trouble.
"He makes her laugh because some of the things he says and does and the way he looks at life is obviously slightly different than her, so together they’re a great couple. “One of the things I know that over the years they’ve loved is when things go wrong – they absolutely adore it because obviously everything always has to be right, but when things go wrong around them they’re the first people to laugh.”