To honour the Dutch monarch's big day, HELLO! Online have gathered a list of ten facts you may not know about the popular royal.
King Willem-Alexander and Queen Maxima
1.The future King was born on 27 April 1967 at the University Medical Center in Utrecht, Netherlands, as the first child of the then Princess Beatrix and her husband Prince Claus.
Upon birth, his official royal title was 'His Royal Highness Prince Willem-Alexander of the Netherlands'.
2. His birth was celebrated across the Netherlands as he was the first Dutch male royal baby born since Prince Alexander 1851, and more importantly the first immediate male heir since Alexander had died in 1884.
3. The Prince studied at three different schools which included spending two years at the United World College of the Atlantic in Wales from 1983 to 1985.
King Willem-Alexander as a baby with his mother Princess Beatrix
Willem-Alexander later admitted spending time apart from his parents helped grow their bond. "I had problems with my parents at the time," said the royal in a TV interview much later. "And my parents had problems with me. So it was best for us to split up."
4. Since the age of 18, he has actively participated in royal duties and the dedicated Willem-Alexander is hugely interested water management and sport issues, across the Netherlands and the world.
He has expressed support for his country to bid for the 2028 Summer Olympics.
5. At the age of 47, he is Europe's youngest monarch.
6. In March 2001, the Dutch Palace announced Willem-Alexander was to marry Maxima Zorreguieta Cerruti, a former investment banker from Argentina, and just 11 months later in February 2002 the Dutch people rejoiced as the new royal couple married in a stunning ceremony at the Nieuwe Kerk in Amsterdam.
7. Willem-Alexander became a father for the first time in December 2003, when his and Maxima's first baby, Princess Catharina-Amalia was born. She was followed by Princess Alexia in June 2005 and Princess Ariane in April 2007.
The prince joked he and Maxima had chosen three names beginning with an A to get a triple A rating.
8. In January 2013 Queen Beatrix announced she would be abdicating in favour of Willem-Alexander, and three months later on 30 April, royals from around the world including Prince Charles and the Duchess of Cornwall, gathered to mark the new King's investiture.
Hundreds of thousands of people gathered outside the Royal Palace of Amsterdam to celebrate their new King. Williem-Alexander ascending the throne was significant in the Netherlands as he became the first King of the Netherlands since the death of his great-great-grandfather King William III in 1890.
The royal couple have three daughters (L-R) Princess Catharina-Amalia, Princess Alexia and Princess Ariane
9. The Dutch royal is a keen sportsman and in 1992 ran the New York City Marathon under the pseudonym 'W. A. van Buren'. He is also an active aircraft pilot, and to ensure he flies enough hours every year to retain his license he sometimes flies the Dutch Royal airplane.
10. Although difficult to count with certainty, Willem-Alexander is in line to the British throne and just before his accession he was estimated to be 889th.
Through his great-great-great-great-great-great-great-grandfather Jan Willem Friso, Prince of Orange, the King is a cousin of many European royals including, Queen Margrethe II of Denmark, former King Albert II of the Belgians, the Grand-Duke of Luxembourg, King Harald V of Norway, King Juan-Carlos of Spain, Prince Albert II of Monaco, and our very own Queen Elizabeth II.