This dissertation is concerned with debates taking place on the Iberian Peninsula during the later Middle Ages over kingship, knighthood, investiture, and administration—questions of sovereignty, power, and social relations endemic to the wider medieval world. With special attention given to the period 1250-1350, the dissertation situates these debates against the backdrop of the dramatic shift in the Peninsula’s balance of power occurring with the Christian conquest of nearly the whole of Muslim Iberia during the early thirteenth century. This process saw the Kingdom of Castile nearly triple its territory and its subject population in the span of four decades, emerging by 1250 as Iberia’s dominant political entity. With no significant Muslim counterweight left on the Peninsula in the wake of the Christian conquests, the holdings and prestige of the Castilian crown were significantly enhanced in a relatively short period of time, but this sudden shift did not translate to broad consensus regarding how Castile’s vastly expanded territory (which was home to sizeable communities of Muslims and Jews) should be governed. This question of administration, and related issues of the relationship of royal authority to the nobility and to Castile’s religious minorities, became perennial concerns of Castilian rulers and members of the aristocracy alike. As such, the dissertation takes these debates as its primary focus, examining not only the processes by which Castilian royal administration was expanded during this period, but also the ways in which that administration and royal authority itself were contested and came to be represented across a range of discursive registers. The dissertation centers on an examination of the administration and historiographical output of Alfonso XI, King of Castile from 1312 until 1350 (his death), offering a new analysis of this relatively little-studied ruler whose political and literary projects took up the question of late medieval sovereignty in novel and significant ways while shaping the exercise and representation of royal power in Castile thereafter. As the dissertation argues, Alfonso imparted a new ideological character to Castile’s royal history and institutions with lasting implications for Spanish and European politics into early modernity. The dissertation intervenes in scholarly conversations surrounding medieval kingship and administration, political rituals and symbols of power, and relations among Christians, Muslims, and Jews.