Watch: King Charles's first address to the nation
Queen Elizabeth II's death has brought the second Elizabethan era to an end.
Having been heir apparent since he was three years old, Charles is now fulfilling his royal destiny and reigning as monarch.
But what this new era will become known as? Here is everything we know so far.
What will King Charles' reign be called?
Prime minister Liz Truss has said King Charles' reign marks a "new era of hope and progress, our new Carolean age".
The term Carolean, which was previously associated with Charles II, is derived from Carolus, the Latin for Charles.
Charles I's time on the throne is usually referred to as the Caroline Age, based on the feminine form of the same adjective.
However, it's possible that future historians could use an entirely different name for the current time period, such as the 'Windsor era'.
Why is the King known as Charles III?
Charles was free to pick his title when he became King.
He could have chosen one of his three middle names – Philip, Arthur or George – but opted for his Christian name, just like his late mother Queen Elizabeth II.
There had been speculation in the past that Charles favoured George VII for historical reasons.
Read more: The Queen by the people who knew her
After the House of Hanover came to power in 1714, Georges – from George I to George IV – ruled for 116 years in a row.
Picking Charles, however, maintains continuity and fortifies the Charles "brand".
The name Charles is seen as historically jinxed by some in royal circles.
The reigns of Charles I (1625-1649) and Charles II (1660-1685) saw the overthrow and restoration of the monarchy, the Great Fire of London, and the Plague.
Charles I is the only British monarch to have been executed for treason, while Charles II was exiled and fathered at least 13 illegitimate children.