The Duke of Cambridge has become the heir apparent following the death of his grandmother, Elizabeth II, and the accession of his father, Charles.
Now first in line to the throne, William’s role within the royal family will change significantly.
As a king-in-waiting, William is a step closer to becoming sovereign himself and preparations for this duty will intensify.
William is decades older than his father was when he became heir to the throne.
He has, comparatively, had a much more gradual introduction to royal duty, spending time as an RAF search and rescue pilot and later as a pilot for the East Anglian Air Ambulance.
William has focused much of his charitable work on mental health, particularly trying to break the stigma surrounding men’s mental health.
He suffered trauma in his childhood with the sudden death of his mother Diana, Princess of Wales, in a car crash, and has faced a rift with younger brother, the Duke of Sussex.
But William has a settled home life with the Duchess of Cambridge and is a hands-on father to their three children, Prince George, Princess Charlotte and Prince Louis.
He was made the Prince of Wales and the Earl of Chester by the Queen in 1958 when he was nine, and was invested as the Prince of Wales in 1969 at the age of 20.
William is expected to, but does not automatically, become the Prince of Wales.
It is up to the new king to decide whether to, or when to, confer the title created for the male heir to the throne.
George is now second in line to the throne, Charlotte third, and Louis fourth, with Harry once again fifth in line.
Kate is a queen in waiting and will play a key role as one of the Windsors’ most senior women.
William’s former university flatmate has been heralded for her “keep calm and carry on” approach to royal life, and she has channelled her energy into work on mental health, the early years of childhood and the benefits of sport and outdoor life.
Both William and Kate are seen as a safe pair of hands in terms of the future of the monarchy.
Charles had talked previously of how he hoped William and Harry would one day take over his youth charity, The Prince’s Trust.
But Harry has quit royal duties and is no longer a senior working royal.
William will have to decide, with his father, whether the transition will go ahead and whether he will take over any of his other causes.
As the 25th Duke of Cornwall, he is entitled to the annual net surplus from the Duchy of Cornwall landed estate – which comes to £23 million a year.
The income will cover the cost of both his public and private life.
He already received money from the duchy through his father, but now, as heir to the throne, he is entitled to take over its management.
The landed estate is valued at more than £1 billion and is one of the largest and oldest in Britain.
It was created in 1337 by Edward III for his son, Edward the Black Prince, and is passed on to provide income from subsequent heirs to the throne.
William will not own the duchy’s assets, but his income is generated from areas such as rent, or dividends on shares.
The Oval cricket ground and Dartmoor Prison are just some of the properties owned by the duchy.
As heir apparent, William has also automatically inherited the titles – under a 1469 Act of the Scottish Parliament – the Duke of Rothesay, Earl of Carrick, Baron of Renfrew, Lord of the Isles and Prince and Great Steward of Scotland.
The Earl of Chester, an earldom created by William the Conqueror, is not automatically inherited, but is usually also given to the heir apparent.