Britain's line of succession has long seen Prince William sit behind his father as a one-day future King.
After King Charles' reign, it will be William who takes the top job - something he has been primed all of his life to do.
But William isn't alone in being an heir to the throne, across Europe and even the world there are a handful of men, all earmarked as future Kings.
So who are these royal princes, who will one day rule their homelands like the Prince of Wales? Here we take a look...
Crown Prince Hussein of Jordan
A close pal of fellow heir to the throne William, Crown Prince Hussein of Jordan is the eldest son of King Abdullah of Jordan and wife Queen Rania.
And he hit the headlines last week when he married his wife Princess Rajwa in a lavish ceremony in Amman fit for a future monarch.
Hussein was just four when his father became King of Jordan and he was named heir apparent in 2004.
He completed his early education in Jordan before gaining a degree in International History at Georgetown University in Washington DC in 2016.
In 2017, he completed his training at the Sandhurst Military Academy in the UK following in the footsteps of his father as well as Princes William and Harry.
He is also a qualified helicopter pilot and has attended events at the United Nations as well as the World Economic Forum.
Many of his fans keep up-to-date with his life on his very active Instagram account, where he has racked up more than four million followers.
On his account, he mixes photos of his official duties with more candid family snaps - and even shared a photo of him with his parents and the late Elizabeth II.
There are also images of him enjoying games of football, riding his motorbike and playing the guitar.
Crown Prince Frederik of Denmark
His mother Queen Margrethe has been monarch of Denmark for over 50 years - and for all of that time, her eldest son Frederik has been her heir.
Like many royal men, he gained a degree in political science at university before serving in Denmark's military and beginning royal duties.
The sporty prince is also the first royal to have ever completed an Ironman challenge and came extremely close to representing Denmark in sailing at the 2016 Olympics.
The 55-year-old met Australian ad executive Mary Donaldson in a Sydney bar during the 2000 Olympics.
Mary didn’t recognise the handsome stranger who introduced himself as “Fred” and gave him her phone number.
He later recalled feeling she was his "soulmate" from that first encounter. When news of their long-distance romance broke, Mary moved to Copenhagen and got a job at Microsoft.
They wed in 2004, with Frederik declaring, “Mary is mine and I am hers.” They now have four children Christian, Isabella, and twins Vincent and Josephine.
Last year, Frederik became embroiled in family drama after his mother Margrethe decided to strip the children of his brother Prince Joachim of their prince and princess titles.
At the time, Joachim and his wife Princess Marie said they were "blindsided" by the news, but Frederik ultimately backed his mother's move calling it "the right thing to do".
Crown Prince Haakon
The heir to the Norwegian throne is the son of King Harald and Queen Sonja - Crown Prince Haakon.
He has a sister, Princess Martha Louise who is older than him - but at the time of Haakon's birth, males took precedence in the line of succession.
The 49-year-old is a trained naval officer and military man and has stepped in as regent on several occasions when his father has suffered ill health.
When it was first reported that he was dating now wife Crown Princess Mette-Marit, eyebrows were raised across Norway.
That's because she was a single mother and was said to have had a "rebellious" youth, part of a rave scene.
She once said: "I certainly didn’t have many ballgowns before I met Haakon!"
They were also a controversial pairing because they moved in together months before their engagement announcement.
However, they wed at Oslo Cathedral in 2001 and have two children Ingrid and Sverre, while Haakon is stepdad to Mette-Marit's first son Marius.
Like Frederik and William, Haakon has had to deal with a sibling, who has also proved controversial.
His sister Martha Louise decided to relinquish any royal duties in 2022 to "differentiate more clearly between her own activities and her relationship to the Royal House of Norway".
It came after she became engaged to Durek Verrett, an American who describes himself as a shaman and a healer.
At the time Haakon was quizzed about his sister and her royal future and admitted it was a "difficult" situation.
Prince Jacques
The heir to the throne of Monaco is among the youngest in the world at just eight years old.
Prince Jacques is a son of current ruler Prince Albert and wife Princess Charlene - and despite being slightly younger than his twin sister Princess Gabriella is heir due to males being given precedence.
Jacques also has an older half-brother and half-sister from his father's previous relationships. However, as Albert never married their mothers, they are not in Monaco's line of succession.
Like young royals in other countries, Jacques and Gabriella have only made rare public appearances. These have included on the balcony of the Prince's Palace on Monaco's national day, which is each December.
The children are homeschooled but Albert once explained they aren't learning alone and a small classroom had been set up for them and some friends.
Two years ago proved a particularly difficult one for young Jacques as he and Gabriella spent large periods of time separated from their mum Charlene.
Charlene travelled to her native South Africa but suffered an ear, nose and throat infection while she was there.
She had to undergo surgery and was unable to travel back to Monaco, forcing her apart from her children.
While appearing on the balcony of Prince's Palace on Monaco's national day that year, her children held up heartrending signs saying: "We love you mummy" and "we miss you mummy".
They were later reunited with Charlene describing the reunion as being "filled with joy and emotion".
