Prince Charles is now the King. Upon the death of his mother Queen Elizabeth today at age 96, Charles immediately ascended to the throne, per the British line of succession.
Just as Queen Elizabeth became monarch the moment her father, King George VI, died, King Charles, too, takes on the role of British monarch immediately.
In the past year, Charles had taken on increased responsibilities on behalf of his ailing mother. In May, he opened parliament for the first time and stepped in for her at the Easter Maundy Service, Remembrance Sunday, and the Commonwealth Day Service.
Like his mother, Charles will have to decide what he wants his regnal name to be. Elizabeth decided to use her birth name, and Charles will likely follow.
The BBC reported Charles will officially be known as King Charles III. It's as yet unclear when Charles's coronation will take place.
King Charles was the longest-serving Prince of Wales in British history. There is no automatic succession to the title, so his heir apparent--his eldest son, Prince William--must be created Prince of Wales
However, it is likely that Prince William will become Prince of Wales and Kate Middleton made Princess of Wales, the title famously held by William's mother Princess Diana.
Earlier this summer, during the Duke and Duchess of Cambridge's trip to Cardiff, Wales during Queen Elizabeth's Platinum Jubilee,
a royal fan told Kate, "You're going to make a brilliant Princess of Wales." Kate replied, "That's very kind. I'm in good hands," gesturing to her husband.