House of Trastámara in the context of "Spanish Kings"

The Catholic Monarchs were Queen Isabella I of Castile (r. 1474–1504) and King Ferdinand II of Aragon (r. 1479–1516), whose marriage and joint rule marked the de facto unification of Spain. They were both from the House of Trastámara and were second cousins, as they were both descended from John I of Castile. To remove the obstacle that this consanguinity would otherwise have posed to their marriage under canon law, they were given a papal dispensation by Sixtus IV. They married on October 19, 1469, in the city of Valladolid; Isabella was 18 years old and Ferdinand a year younger. Most scholars generally accept that the unification of Spain can essentially be traced back to the marriage of Ferdinand and Isabella. Their reign was called by W.H. Prescott "the most glorious epoch in the annals of Spain."

Spain was formed as a dynastic union of two crowns rather than a unitary state, as Castile and Aragon remained separate kingdoms until the Nueva Planta decrees of 1707–1716. The court of Ferdinand and Isabella was constantly on the move in order to bolster local support for the crown from local feudal lords. The title of "Catholic King and Queen" was officially bestowed on Ferdinand and Isabella by Pope Alexander VI in 1494, in recognition of their defence of the Catholic faith within their realms.

The House of Barcelona was a medieval dynasty that ruled the County of Barcelona continuously from 878 and the Crown of Aragon from 1137 (as kings from 1162) until 1410. They descend from the Bellonids, the descendants of Wilfred the Hairy. They inherited most of the Catalan counties by the thirteenth century and established a territorial Principality of Catalonia, uniting it with the Kingdom of Aragon through marriage and conquering numerous other lands and kingdoms until the death of the last legitimate male of the main branch, Martin the Humanist, in 1410. Cadet branches of the house continued to rule Urgell (since 992) and Gandia. Cadet branches of the dynasty had also ruled Ausona intermittently from 878 until 1111, Provence from 1112 to 1245, and Sicily from 1282 to 1409. By the Compromise of Caspe of 1412 the Crown of Aragon passed to a branch of the House of Trastámara, descended from the infanta Eleanor of the house of Barcelona.

This is a list of Spanish heads of state, that is, monarchs and presidents that governed the country of Spain in the modern sense of the word. The forerunners of the Spanish throne were the following:

• Kings of Asturias
• Kings of Navarre
• Kings of León
• Kings of Galicia
• Kings of Aragon
• Kings of Castile

These lineages were eventually united by the marriage of the Catholic Monarchs, Ferdinand II of Aragon (king of the Crown of Aragon) and Isabella I of Castile (queen of the Crown of Castile). Although their kingdoms continued to be separate, with their personal union they ruled them together as one dominion. Spain was thereafter governed as a dynastic union by the House of Trastámara, the House of Habsburg, and the House of Bourbon until the Nueva Planta decrees merged Castile and Aragon into one kingdom.

John I (Spanish: Juan I; 24 August 1358 – 9 October 1390) was King of Castile and León from 1379 until 1390. He was the son of Henry II and of his wife Juana Manuel of Castile. John ascended to the throne in 1379 and in 1383, he married Beatrice, the daughter of King Ferdinand I of Portugal. When Ferdinand died that same year, John, aiming to enforce his claim on the Portuguese crown through his wife, brought the country into the 1383–1385 Crisis. His forces faced resistance from Portuguese factions supporting John of Aviz. The conflict culminated in the Battle of Aljubarrota in 1385, where John suffered a defeat, ensuring Portugal's independence.

To secure Castile, he married his son Henry to the daughter of John of Gaunt, linking the Trastámara and Plantagenet dynasties. He died in 1390 when he fell from his horse during a fantasia performance. His death led to minority rule under his son, Henry III.

Duke of Gandía (Valencian: Ducat de Gandia, IPA: [duˈkad de ɣanˈdi.a]) is a title of Spanish nobility that was first created in 1399 by Martin of Aragon and granted to Alfonso of Aragon and Foix. It has its origin in the lordship of Gandía created in 1323 by James II of Aragon. Later, having no direct descendants, the title passed from the House of Barcelona to the House of Trastámara.

The title was re-established in 1483 by Ferdinand II of Aragon as a favour to Rodrigo Cardinal Borgia for his son Pier Luigi Borgia. The dukedom then went to Pier Luigi's half-brother Giovanni Borgia. He was assassinated, and his young son inherited the title. The fourth duke was the religious figure Francesco Borgia. After the death of his wife, with whom he had a large family, he became a Jesuit.

James II (in Catalan Jaume II d'Urgell or Jaume el Dissortat ("James the unlucky"), in Spanish Jaime II el desafortunado) (1380 – 1 June 1433) was the Count of Urgell (1408–1413), Viscount of Àger, and lord of Antillón, Alcolea de Cinca, and Fraga. Scion of a younger branch of the House of Barcelona and its last male member, he was the centre of opposition to the House of Trastámara after it succeeded to the Crown of Aragon in 1412.

Born at Balaguer to Peter II of Urgell and Margaret Palaiologue of Montferrat, James inherited the county of Urgell from his father in 1408. In Valencia on 29 June 1407, he had married Isabella, daughter of Peter IV of Aragon, who appointed him lieutenant of the Kingdom of Aragon in 1408.