Old Spanish, also known as Old Castilian, or Medieval Spanish, was originally a dialect of Vulgar Latin spoken in the former provinces of the Roman Empire. It provided the root for the early form of the Spanish language that was spoken on the Iberian Peninsula from the 9th century until roughly the beginning of the 15th century, before a series of consonant shifts gave rise to modern Spanish. The poem Cantar de Mio Cid, published around 1200, is the best known and most extensive work of literature in Old Spanish.

El Cantar de mio Cid, or El Poema de mio Cid, also known in English as The Poem of the Cid, is the oldest preserved Castilian epic poem. Based on a true story, it tells of the deeds of the Castilian hero Rodrigo Díaz de Vivar—known as El Cid—and takes place during the eleventh century, an era of conflicts in the Iberian Peninsula between the Kingdom of Castile and various Taifa principalities of Al-Andalus. It is considered a national epic of Spain.