The Queen has now been on the throne for 66 years, and shows few signs of slowing down now.
The 92-year-old is the longest-reigning monarch in the UK's history. She has seen 13 Prime Ministers pass through Downing Street, and witnessed the passing of the British Empire and the rise of the Commonwealth.
But with her husband Prince Philip retiring from public duties in 2017, and her son Prince Charles taking on more of her overseas duties, people are looking at what could happen when the crown passes on.
While there is an element of unpredictability about how the world would react, many elements have been planned meticulously and are likely to bring unprecedented scenes to the UK.
From changes to the national anthem, stock market woes and a suspension of comedy shows, here are some of the things we can expect when the Queen dies...
National mourning
The Queen's death will trigger an official 12-day period of national mourning.
Union Jacks will be flown at half mast throughout the country and books of condolence will be opened at embassies across the world.
The London Stock Exchange will be closed for at least the day of the Queen's funeral and potentially for several days - potentially costing the economy billions.
The Queen’s body will be taken to Parliament, where she will lie in state in Westminster Hall – like the Queen Mother in 2002 – until her funeral.
More than 200,000 people travelled to pay their respects to the Queen Mother and that number is expected to be vastly exceeded in the event of the Queen's death.
Media coverage
The licence fee-funded BBC will suspend all programming and screen BBC One coverage of the event.
If Her Majesty dies during the night, her death would reportedly not be announced until 8am, after which her portrait will be displayed on the screen accompanied by the national anthem - as happened following the death of Princess Diana in 1997.
Other channels are not required to interrupt regular scheduling, but almost certainly will.
The BBC will also suspend all comedy programming during the 12-day period of national mourning.
The Beeb regularly practices how it will respond to the Queen's death.
It was during one practice exercise when a BBC Urdu journalist mistakenly tweeted that the Queen had died.
The succession
As first in line to the throne, Prince Charles will automatically become King when the Queen dies as the throne is technically never vacant.
However, the accession council would need to be convened at St James’s Palace to formally declare Charles the king.
He would be asked to swear loyalty to Parliament and the Church of England in front of the council.
In turn, the Houses of Parliament will be recalled to swear loyalty to him.
The words to the National Anthem will be changed and new postage stamps and currency created to reflect Prince Charles’ ascension to the throne.
The only way Prince William would become king is if Prince Charles chooses to abdicate and pass the throne to him.
Charles has increased his workload since 2013 as the Queen's has reduced in what appears to be a handover of the reins.
However, in 2016 found that only 53 per cent of people say they like Charles, compared to 77 per cent for the Queen.
The funeral
The Queen's state funeral will be the biggest ever seen in the UK - far eclipsing the likes of Princess Diana, the Queen Mother and Margaret Thatcher.
The service will be led by the Archbishop of Canterbury, currently the Rev Justin Welby.
Politicians and heads of state from across the globe will attend as well as the Royal Family.
As such, the event is likely to be one of the most heavily-protected of all time, with armed forces drafted in to guard against potential terror attacks.
Her coffin will be taken from Westminster Hall and travel through crowds of thousands of mourners by gun carriage to Westminster Abbey for the service.
It is expected the Queen will be interred in St George's Chapel at Windsor Castle, where her mother and father, King George VI, are buried.