Charles III became King in September 2022 following the death of his mother, Queen Elizabeth II.
He was the longest-serving heir apparent and Prince of Wales, and at the age of 73, he became the oldest person to accede to the British throne.
Charles joins a long list of reigning monarchs around Europe, but do you know who is the current longest-serving king or queen? Some are also former Olympians while others are set to pass milestones in 2023. Learn more in the video below...
WATCH: Meet the Kings and Queens of Europe
Here's everything you need to know about the current reigning monarchs in Europe.
Queen Margrethe of Denmark
Reigned since: 14 January 1972
Heir: Crown Prince Frederik
Queen Margrethe celebrated her Golden Jubilee in 2022 and is currently Europe's longest-serving current head of state as well as the only reigning queen.
The Danish monarch is mother to Crown Prince Frederik and Prince Joachim, and grandmother to eight grandchildren.
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Last year, it announced that Queen Margrethe would be removing four of her grandchildren's HRH titles. The changes for Prince Joachim's children, Nikolai and Felix – his sons with his first wife, Alexandra – and Athena and Henrik, who he shares with Princess Marie, came into effect in January.
Margrethe made a public apology about her decision, and in a recent interview she said: "For me, it has been important that it should not be Frederik's lot to make such a decision. It was better that it was me."
King Carl XVI Gustaf of Sweden
Reigned since: 15 September 1973
Heir: Crown Princess Victoria
Carl Gustaf ascended the Swedish throne in 1973 following the death of his grandfather King Gustaf VI Adolf.
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There was heartache for the king in his early childhood as his father, Prince Gustaf Adolf, died in an airplane crash in Denmark when he was just nine months old.
King Carl XVI Gustaf and his wife, Queen Silvia, share three children, Crown Princess Victoria, Prince Carl Philip and Princess Madeleine, as well as eight grandchildren.
Prince Carl Philip was born as the heir apparent but when absolute primogeniture was introduced in 1980, his elder sister, Victoria, then became heir.
The king objected to the constitutional reform and in a recent interview, he said he believed it to be unfair that his son lost his status as Crown Prince.
King Harald V of Norway
Reigned since: 17 January 1991
Heir: Crown Prince Haakon
A keen sportsman, Harald represented Norway in sailing at the 1964, 1968 and 1972 Olympic Games.
King Harald and his wife Sonja share two children, Princess Martha Louise and Crown Prince Haakon, as well as five grandchildren.
Grand Duke Henri of Luxembourg
Reigned since: 7 October 2000
Heir: Prince Guillaume, Hereditary Grand Duke of Luxembourg
Grand Duke Henri became head of state in 2000, following his father Grand Duke Jean's abdication.
He married Cuban-born Maria Teresa Mestre y Batista in 1981, and the couple share five children.
Prince Albert II of Monaco
Reigned since: 6 April 2005
Heir: Prince Jacques
The only son of Prince Rainier III and American actress, Grace Kelly, Albert competed in bobsleigh during the Winter Olympic finals before retiring in 2002.
He married South African Olympic swimmer, Charlene Wittstock, in 2011, and the couple welcomed their twins, Princess Gabriella and Prince Jacques, in December 2014.
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Prince Albert has also fathered two other children – Jazmin Grace Grimaldi and Alexandre Grimaldi-Coste, who are not in the line of succession to the Monegasque throne.
King Willem-Alexander of The Netherlands
Reigned since: 30 April 2013
Heir: Princess Catharina-Amalia of Orange
Willem-Alexander became king following his mother Beatrix's abdication in 2013.
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He married Argentine-born Maxima Zorreguieta Cerruti in 2002, and the couple share three daughters – Princess Catharina-Amalia, Princess Alexia and Princess Ariane.
King Philippe of Belgium
Reigned since: 21 July 2013
Heir: Princess Elisabeth, Duchess of Brabant
King Philippe is the first cousin of Grand Duke Henri of Luxembourg. He succeeded his father King Albert II upon the latter's abdication for health reasons in 2013.
Philippe shares four children with wife, Queen Mathilde – Princess Elisabeth, Prince Gabriel, Prince Emmanuel and Princess Eleonore.
King Felipe of Spain
Reigned since: 19 June 2014
Heir: Princess Leonor of Asturias
Felipe became Spain's head of state following his father Juan Carlos I's abdication in 2014.
Upon his accession, he became the youngest monarch in Europe, being nine months younger than King Willem-Alexander of the Netherlands.
King Felipe shares two daughters, Princess Leonor and Infanta Sofia, with wife and former journalist, Letizia Ortiz.
King Charles of the UK
Reigned since: 8 September 2022
Heir: The Prince of Wales
Charles became heir apparent at the age of four when his mother, Elizabeth II, became Queen in 1952.
He has two sons, Prince William and Prince Harry, from his first marriage to Diana, Princess of Wales, who was tragically killed in a car crash in 1997.
Charles married his longtime love, Camilla Parker Bowles, in 2005.
The couple's joint coronation will take place on 6 May 2023 at Westminster Abbey.
LISTEN: King Charles' adventures from the cockpit while piloting planes