The Cantar de Mio Cid, the oldest preserved Castilian epic poem, was composed around 1200, recounting the exploits of Rodrigo Díaz de Vivar (El Cid), an 11th-century Castilian nobleman and mercenary. The poem blends historical fact with legendary embellishment, portraying the Cid as an ideal knight loyal to his king despite unjust exile.

The Cantar de Mio Cid established foundational themes for Spanish literature, including honor, loyalty, and social mobility through merit rather than birth. Its realistic portrayal of frontier life and complex Christian-Muslim relations distinguished it from French chansons de geste's more schematic religious oppositions. The poem's irregular verse form and formulaic style reflect oral compositional techniques, preserving connections to epic performance traditions while transitioning toward written literature.