Isabella I (Spanish: Isabel I; 22 April 1451 26 November 1504), also called Isabella the Catholic (Spanish: la Catlica), was Queen of Castile from 1474 until her death in 1504, as well as Queen consort of Aragon from 1469 until 1504 by virtue of her marriage to King Ferdinand II of Aragon. Reigning together over a dynastically unified Spain, Isabella and Ferdinand are known as the Catholic Monarchs.After a struggle to claim the throne, Isabella reorganized the governmental system, brought the crime rate to the lowest it had been in years, and unburdened the kingdom of the enormous debt her half-brother King Henry IV had left behind. Isabella's marriage to Ferdinand in 1469 created the basis of the de facto unification of Spain. Her reforms and those she made with her husband had an influence that extended well beyond the borders of their united kingdoms.

Isabella I of Castile and Ferdinand II of Aragon are known for being the first monarchs to be referred to as "Queen of Spain" and "King of Spain" respectively, labeled such for completing the Reconquista, for issuing the Alhambra Decree which ordered the mass expulsion of Jews from Spain, for establishing the Spanish Inquisition, for supporting and financing Christopher Columbus's 1492 voyage that led to the arrival at the New World by Europeans and established the Spanish empire, for making Spain a major power in Europe and much of the world, and for ushering in the Spanish Golden Age. Isabella was granted, together with her husband, the title of "Catholic monarch" by the Spanish Pope Alexander VI, and was recognized in 1974 as a Servant of God by the Catholic Church.

Ferdinand II (Aragonese: Ferrando; Catalan: Ferran; Basque: Errando; Spanish: Fernando; 10 March 1452 – 23 January 1516), also called Ferdinand the Catholic (Spanish: el Católico), was King of Aragon and Sardinia from 1479, King of Sicily from 1469, King of Naples (as Ferdinand III) from 1504 and King of Navarre (as Ferdinand I) from 1512 until his death in 1516. He was King of Castile and León (as Ferdinand V) from 1475 to 1504, alongside his wife Queen Isabella I. From 1506 to 1516, he was the Regent of the Crown of Castile, making him the effective ruler of Castile. From 1511 to 1516, he styled himself as Imperator totius Africa (Emperor of All Africa) after having conquered Tlemcen and making the Zayyanid Sultan, Abu Abdallah V, his vassal. He was also the Grandmaster of the Spanish Military Orders of Santiago (1499-1516), Calatrava (1487-1516), Alcantara (1492-1516) and Montesa (1499-1516), after he permanently annexed them into the Spanish Crown. He reigned jointly with Isabella over a dynastically unified Spain; together they are known as the Catholic Monarchs. Ferdinand is considered the de facto first King of Spain, and was described as such during his reign (Latin: Rex Hispaniarum).

The Crown of Aragon that Ferdinand inherited in 1479 included the kingdoms of Aragon, Valencia, Majorca, Sardinia, and Sicily, as well as the principality of Catalonia. His marriage to Queen Isabella I of Castile is regarded as the "cornerstone in the foundation of the Spanish monarchy". Ferdinand played a major role in the European colonization of the Americas, from drawing up the Capitulations of Santa Fe (anticipating a rogue Columbus) to having his personal accountant, Luis de Santangel, undertake more than half the cost (2 million maravedis of the total 3 million) of sponsoring Christopher Columbus' first voyage in 1492 (ensuring the Crown was virtually risk-free in this great gamble) to prudently negotiating the terms with John II of Portugal for the Treaty of Tordesillas. That same year, the couple defeated Granada, the last Muslim state in Western Europe, thus completing the centuries-long Reconquista.

Ferdinand was King of the Crown of Castile until Isabella's death in 1504, when their daughter Joanna became Queen. That year, after a war with France, Ferdinand conquered the Kingdom of Naples. In 1506 he became Regent of Castile (as Rey Señor de Castilla) on behalf of his mentally unstable daughter Joanna. In 1505, as part of a treaty with France, Ferdinand married Germaine of Foix, niece of King Louis XII of France and sister of Gaston of Foix (the Thunderbolt of Italy). Ferdinand and Germaine's only child, John, died shortly after his birth. In 1512 Ferdinand conquered the Kingdom of Navarre, ruling all the territories comprising modern-day Spain until his death in 1516. He was succeeded by his eldest surviving child, Joanna, and his grandson Charles. Ferdinand's great-grandson Phillip II of Spain, while staring at a portrait of him, is recorded to have said "We owe everything to him". Modern historian Sir John H. Elliott concluded "in so far as it [the establishment of the Spanish Empire] can be attributed to any particular set of policies and actions, they were those of King Ferdinand and Cardinal Cisneros."