Prince William & Kate's Last Name: Mountbatten-Windsor

Now that Prince William and Kate Middleton are married, some are wondering their last name.

Like other descendants of Queen Elizabeth II and Prince Philip, William's surname is Mountbatten-Windsor, and it will be Kate's too by marriage.

As explained by the official British royal website, the Queen's descendants carry the name Mountbatten, as well as the royal surname Windsor, to reflect her husband Prince Philip's surname.

The royal website explains further that the surnames are not always used, only when they are formally needed, such as the case of marriage.

While Prince William will be recorded as a Mountbatten-Windsor in the marriage registry today, he may choose a new name at a later point, specifically if and when he becomes king. As explained here, "Since a royal proclamation is not binding on succeeding rulers, King William could decide to add a name or two."

Prince William and Kate, who will go by the name of Catherine, also inherited the titles Duke and Duchess of Cambridge upon their marriage today.
source: special