Joanna, historically known as Joanna the Mad, was the nominal queen of Castile from 1504 and queen of Aragon from 1516 to her death in 1555. She was the daughter of Queen Isabella I of Castile and King Ferdinand II of Aragon. Joanna was married by arrangement to the Austrian archduke Philip the Handsome on 20 October 1496. Following the deaths of her elder brother John, elder sister Isabella, and nephew Miguel between 1497 and 1500, Joanna became the heir presumptive to the crowns of Castile and Aragon. When her mother died in 1504, she became queen of Castile. Her father proclaimed himself governor and administrator of Castile.

Isabella I, also called Isabella the Catholic, was Queen of Castile and León from 1474 until her death in 1504. She was also Queen of Aragon from 1479 until her death as the wife of King Ferdinand II. Reigning together over a dynastically unified Spain, Isabella and Ferdinand are known as the Catholic Monarchs.