This is a list of counts of Castile. The County of Castile had its origin in a fortified march on the eastern frontier of the Kingdom of Asturias. The earliest counts were not hereditary, being appointed as representatives of the Asturian king. From as early as 867, with the creation of the County of Álava, Castile was subdivided into several smaller counties that were not reunited until 931. In the later 10th-century, while nominally in vassalage to the Kingdom of León, the counts grew in autonomy and played a significant role in Iberian politics. After the assassination in 1029 of Count García Sánchez of Castile, King Sancho III of Pamplona, because of his marriage to Muniadona, García's sister, governed the county although he never held the title of count: it was his son, Ferdinand Sánchez, the future King Ferdinand I of León who inherited the county from his mother.
Near the end of 1063,[a] Fernando I convened the Curia regis to announce his testamentary dispositions pursuant to which he had decided to divide his dominions among his sons. His firstborn, Sancho, became the first king of the former county and reigned as Sancho I of Castile.