Ferdinand I (10 March 1503, Alcalá de Henares, Spain – 25 July 1564, Vienna, Habsburg domain [now in Austria]) was Holy Roman Emperor from 1558, king of Bohemia and Hungary from 1526, and king of Croatia from 1527 until his death. Before his accession, he ruled the Austrian hereditary lands of the Habsburgs in the name of his elder brother, Charles V, Holy Roman Emperor.

The key events during his reign were the contest with the Ottoman Empire, whose great advance into Central Europe began in the 1520s, and the Protestant Reformation, which resulted in several wars of religion.

Ferdinand's motto was Fiat iustitia, et pereat mundus: "Let justice be done, though the world perish".

Ferdinand was born in Alcalá de Henares, Spain, the son of the Trastamara Infanta Joanna ("Joanna the Mad"), and Habsburg Archduke Philip the Handsome, who was heir to Maximilian I, Holy Roman Emperor. Ferdinand shared his birthday with his maternal grandfather Ferdinand II of Aragon.

On the accession of his brother Charles to title of Holy Roman Emperor in 1519, he entrusted Ferdinand with the government of the Austrian hereditary lands, roughly modern-day Austria and Slovenia. Ferdinand was Archduke of Austria from 1521 to 1564.

The Holy Roman Emperor (German: Römisch-Deutscher Kaiser, or "Roman-German Emperor" Latin: Imperator Romanus Sacer) is a term used by historians to denote a medieval ruler who had also received the title of "Emperor of the Romans" from the Pope. After the 16th century, this elected monarch governed the Holy Roman Empire (later called Holy Roman Empire of the German Nation), a Central European union of territories of the Medieval and Early Modern period.

The title of Emperor (Imperator) carried with it an important role as protector of the Catholic Church. As the papacy's power grew during the Middle Ages, Popes and emperors came into conflict over church administration. The best-known and bitterest conflict was that known as the Investiture Controversy, fought during the 11th century between Henry IV and Pope Gregory VII.

After Charlemagne was crowned Roman Emperor by the Pope, his successors maintained the title until the death of Berengar I of Italy in 924. No pope appointed an emperor again until the coronation of Otto the Great in 962. Otto is considered the first Holy Roman Emperor, although Charlemagne is also accounted by some to be the first. Under Otto and his successors, much of the former Carolingian kingdom of Eastern Francia became the Holy Roman Empire. The various German princes elected one of their peers as King of the Germans, after which he would be crowned as emperor by the Pope. After Charles V's coronation, all succeeding emperors were legally emperors-elect due to the lack of papal coronation, but for all practical purposes they were simply called emperors.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.

Ferdinand the Catholic (Spanish: Fernando II, Catalan: Ferran II, Aragonese: Ferrando II; 10 March 1452 – 23 January 1516) was King of Aragon (as Ferdinand II), Sicily, Naples (as Ferdinand III), Majorca, Valencia, Sardinia, and Navarre, Count of Barcelona, jure uxoris King of Castile (1474–1504, as Ferdinand V, in right of his wife, Isabella I) and then regent of that country also from 1508 to his death, in the name of his mentally unstable daughter Joanna.

Ferdinand was born in Sos del Rey Católico, Aragon, and was the son of John II of Aragon (whose family was a cadet branch of the House of Trastámara) by his second wife, Juana Enríquez. He married Infanta Isabella, the half-sister and heiress of Henry IV of Castile, on 19 October 1469 in Valladolid. Isabella also belonged to the royal House of Trastámara, and the two were second cousins by descent from John I of Castile. They were married with a clear prenuptial agreement on sharing power, and under the joint motto "tanto monta, monta tanto". He became jure uxoris King of Castile when Isabella succeeded her deceased brother in 1474 to be crowned as Queen Isabella I of Castile. The two young monarchs were initially obliged to fight a civil war against Joan of Castile (also known as Juana la Beltraneja), the purported daughter of Henry IV, and were swiftly successful. When Ferdinand succeeded his father as King of Aragon in 1479, the Crown of Castile and the various territories of the Crown of Aragon were united in a personal union creating for the first time since the 8th century a single political unit referred to as España (Spain), the root of which is the ancient name Hispania. The various states were not formally administered as a single unit, but as separate political units under the same Crown. The legal merging of Aragon and Castile into a single Spain occurred under Philip V.

Joanna (6 November 1479 – 12 April 1555), nicknamed Joanna the Mad (Spanish: Juana la Loca), was the first queen regnant to reign over both the Crown of Castile (1504–55) and the Crown of Aragon (1516–55), a union which evolved into modern Spain. Besides the kingdoms of Spain, she also ruled the kingdoms of Sardinia, Sicily, and Naples in Italy; a vast colonial empire in the Americas; and was Countess of Burgundy and the consort of the Burgundian Netherlands, thus initiating Spanish interests there. She was the last monarch of the House of Trastámara and her marriage to Philip the Handsome initiated the rule of the Habsburgs in Spain. Throughout most of her long reign she was under the regency of either her husband, father or son, and she was long confined to a nunnery under the pretense of mental illness^{[citation needed]}.

Joanna was born in the ancient Visigothic city of Toledo, the capital of the Kingdom of Castile. She was the third child and second daughter of Isabella I of Castile and Ferdinand II of Aragon of the royal House of Trastámara. Joanna was an intelligent child and student. In the Castilian court her main tutors were the Dominican priest Andrés de Miranda, the respected educator Beatriz Galindo who was a member of the queen's court, and her mother the queen. She was accomplished in religious studies, court etiquette, the arts of dance and music, and equestrian skills. Joanna mastered all of the Iberian Romance languages: Castilian, Leonese, Galician-Portuguese and Catalan. She also was fluent in French and Latin. She was trained and educated for a significant marriage that, as a royal family alliance, would extend the kingdoms' power and security as well as its influence and peaceful relations with other ruling powers. As an infanta she was not expected to be heiress to the throne of either Castile or Aragon, although through deaths she later inherited both.