The Queen is Britain's longest reigning monarch, having been on the throne for over 70 years. In fact, Her Majesty celebrated her Platinum Jubilee earlier this year.
She became Queen on February 6, 1952, following the death of her father King George VI, on the same date - just months away from her 26th birthday.
In the event of the Queen retiring or passing away, Prince Charles is heir to the British throne and will become King. He is followed by his son Prince William, who is next in line for succession to the Crown.
Until 2013, the succession rules had meant that succession went to the next male heir - meaning that a younger son can displace an elder daughter in the line of succession.
Follow the latest updates on the Queen's health today in our live blog
However in 2013, the Queen did away with the antiquated rule, with the Succession to the Crown Act.
This means that succession now goes to the next member of the Royal Family in birth order, applying to anyone who was born after 2011.
This means that Princess Charlotte, the daughter of the Duke and Duchess of Cambridge, will be the first female heir who is not displaced by her younger brother ( Prince Louis ).
The Royal family's website has a full list of succession - we take a look at the 10 members of the Royal Family who are currently in line for the throne should the Queen retire or pass away.
- The Prince of Wales (Prince Charles)
- The Duke of Cambridge (Prince William)
- Prince George of Cambridge
- Princess Charlotte of Cambridge
- Prince Louis of Cambridge
- The Duke of Sussex (Prince Harry)
- Master Archie Mountbatten-Windsor
- Miss Lilibet Mountbatten-Windsor
- The Duke of York (Prince Andrew)
- Princess Beatrice, Mrs. Edoardo Mapelli Mozzi
Her Majest's reign has outlasted that of those before her, exceeding the likes of Queen Victoria and King George III by several years.
Earlier this year the Queen celebrated her Platinum Jubilee - and although celebrations were in June, it is actually February 6 that marks the official anniversary of when she ascended the throne.
