prince of Asturias, son of King Philip II of Spain and Maria of Portugal, heir to the Spanish throne, whose hatred for his father led him to conspire with the king's enemies in the Low Countries, thus provoking his arrest. His death contributed to the Black Legend of Philip II.

Don Carlos spent his first years at Alcalá de Henares with his aunts, the infantas Doña María and Doña Juana. Except for a short period, the prince did not see his father until he was 14 years of age. In 1554 Philip II entrusted his son's education to Honorato Juan, but the humanist accomplished very little. Don Carlos was sickly and soon showed signs of mental instability, being given to outbursts of violence. In 1560 the Cortes (parliament) of Castile recognized him as heir to the throne, but Philip subsequently decided he was incapable of ruling and barred him from succession to the throne. In 1565 Don Carlos attempted to escape to Flanders and, two years later, to Germany. Finally, Philip II ordered his arrest in January 1568 when he learned of the intrigues of the prince with the marquis of Berghes and the baron of Montigny, nobles involved in the rebellion of the Low Countries. Don Carlos died in prison a few months later. Although his death occurred under mysterious circumstances, there is no evidence that he was executed by order of his father. The accounts that the prince was subject to a judicial process or that his death was caused by his love for Queen Isabella of Valois, Philip's wife, or by his Protestant inclinations lack historical foundation. Don Carlos is probably best known as the hero of a tragedy by Friedrich von Schiller (Schiller, Friedrich von) and of Giuseppe Verdi's opera Don Carlo.