With more than 64 years on the throne, Queen Elizabeth II is the longest reigning monarch in British History. When the Queen dies her son, Prince Charles, will take over (barring any unforeseen circumstances).
The line of succession then goes Prince William, Prince George, Princess Charlotte and then Prince Harry.
The succession line, in fact, responds to the Acts of Settlement which gave precedence to sons of the monarchy over daughters. However, this changed in 2013 with the Succession to the Crown Act.
Produced by Claudia Romeo. Original reporting by Rob Price.