Ferdinand I (1503-1564) was the son of Philip I of Castile and Queen Joanna I of Castile, grandson of Emperor Maximilian I and younger brother of future emperor Charles V. Born and raised in Spain, he was sent to Flanders in 1518. When Charles became Holy Roman emperor in 1519, Ferdinand was entrusted with the governing of their hereditary Austrian lands, becoming Archduke of Austria and adopting the German culture as his own.

Ferdinand became King of Bohemia and Hungary in 1526 after the death of his brother-in-law Louis II, and served as his brother Charles' deputy in the Holy Roman Empire during his numerous absences, eventually being crowned as King of Germany in 1531 and serving as Charles' designated imperial heir. The crown of Croatia also became his in 1527, and fought to push back the Ottomans from Central Europe, eventually repelling them in 1533 but forced to concede the eastern portion of Hungary.

Ferdinand also had to deal with the Protestant Reformation under Luther, and was able to order the Diet in Augsburg, leading to the Peace of Augsburg in 1555. When Charles V abdicated in 1556, Ferdinand was elected as his successor to the imperial throne, becoming Holy Roman emperor in 1558. He continued to ably rule the Holy Roman Empire until his death in 1564, leaving an enduring legacy from his handling of the Protestant Reformation and his efforts against the Ottoman Empire.