Rodrigo Díaz de Vivar (c. 1044 – July 1099), nicknamed El Cid Campeador, was a Castilian military and political leader in medieval Spain. Born of the Spanish nobility, El Cid was educated in the royal Castilian court and became an important general and administrator, fighting against the Moors in the early Reconquista. Later exiled by King Alfonso VI, El Cid left service in Castile and worked as a mercenary-general for other rulers, both Moor and Christian. Late in life, El Cid captured the Mediterranean coastal city of Valencia, ruling it until his death in 1099.

"El Cid Campeador" is a compound of two separate sobriquets. The "El Cid" is derived from the word al-sidi in the Andalusi Arabic dialect (from the Arabic sayyid "sir" or "lord," a title of respect) while the title el campeador (the champion) was granted by his Christian admirers. It is also a not so common fact that he has been referred to by some Andalucians as "El Raffi". These titles reflected the great esteem El Cid had among both Moors and Christians, as well as his fighting ability; Henry Edwards Watts wrote that el campeador "[m]eans in Spanish something more special than "champion" ... A campeador was a man who had fought and beaten the select fighting-man of the opposite side in the presence of the two armies."

Historical records show that El Cid's father was Diego Laínez, who was part minor nobility (infanzones) of Castile. Diego Laínez was a courtier, bureaucrat, and cavalryman who had fought in several battles. Despite the fact in later years the peasants would consider him one of their own, El Cid's mother's family was aristocratic. However, his relatives were not major court officials: documents show that El Cid's paternal grandfather, Lain Nuñez, only confirmed five documents of Ferdinand I's; his maternal grandfather, Rodrigo Alvarez, certified only two of Sancho II's; the Cid's own father confirmed only one. This seems to indicate that El Cid's family was not comprised of major court officials.

One well-known legend about the Cid describes how he acquired his famous war-horse, the white stallion Babieca. According to this story, Rodrigo's godfather, Pedro El Grande, was a monk at a Carthusian monastery. Pedro's coming-of-age gift to El Cid was his pick of a horse from an Andalusian herd. El Cid picked a horse that his godfather thought was a weak, poor choice causing the monk to exclaim "Babieca!" (stupid!) Hence, it became the name of El Cid's horse. Today, Babieca appears in multiple works about El Cid.

El Cid was educated in the Castilian royal court, serving the prince and future king Sancho II, the son of King Ferdinand I ("the Great"). When Ferdinand died in 1065, he had continued his father's goal of enlarging his territory, conquering the Christian and the Moorish cities of Zamora and Badajoz.

By this time, the Cid was a full adult. He had, in 1067, fought alongside Sancho against the Moorish stronghold of Zaragoza (Saragossa), making its emir al-Muqtadir a vassal of Sancho. In the spring of 1063, he fought in the Battle of Graus, where Ferdinand's half-brother, Ramiro I of Aragon, had laid siege to the Moorish town of Graus which was in Zaragozan lands. Al-Muqtadir fought against the Aragonese, accompanied by a Castillian troops, which included the Cid. The party would emerge victorious, Ramiro I was killed, and the Aragonese fled the field. One legend has said that during the conflict El Cid killed an Aragonese knight in single combat, giving him the honorific title of "El Cid Campeador".

Battle tactics
During his campaigns, the Cid often ordered that books by classic Roman and Greek authors on military themes be read in loud voices to him and his troops, both for entertainment and inspiration during battle. El Cid's army had a novel approach to planning strategy as well, holding what might be called brainstorming sessions before each battle to discuss tactics. They frequently used unexpected strategies, engaging in what modern generals would call psychological warfare; waiting for the enemy to be paralyzed with terror and then attacking them suddenly, distracting the enemy with a small group of soldiers, etc. El Cid had a humble personality and frequently accepted or included suggestions from his troops. He remained open to input from his soldiers and to the possibility that he himself was capable of error. The man who served him as his closest adviser was his nephew, Alvar Fáñez de Minaya.

Marriage and family life
The Cid was married in July 1074 to Alfonso's kinswoman Jimena de Gormaz (spelled Ximena in Old Castillian), the daughter of the Count of Oviedo. This was probably on Alfonso's suggestion, a move that he probably hoped would improve relations between him and the Cid. Together the Cid and Ximena had three children. Their daughters, Cristina and María, both married high nobility; Cristina, to Ramiro, lord of Monzón and bastard descendant of kings of Navarre; María, first to Infante of Aragon and second to Ramón Berenguer III, count of Barcelona. The Cid's son, Diego Rodríguez, was killed while fighting against the invading Muslim Almoravids from North Africa at the Battle of Consuegra (1097). His own marriage and that of his daughters increased his status by connecting the Cid to royalty; even today, living monarchs descend from El Cid, through the lines of Navarre and Foix.