This thesis explores narratives of ruling queenship in medieval Castilian histories, tracing the evolving treatment of female rulers through a broad range of Latin and vernacular texts from the twelfth to fifteenth centuries. The investigation focuses on diverse accounts of four queens who exercised royal authority in the kingdom of Castile in the High and Late Middle Ages: Urraca of León-Castile (regnant 1109-1126), Berenguela of Castile (regent 1214-1217 and co-regnant 1217-1246), María de Molina (regent 1295-1301 and 1306-1321), and Catalina de Lancaster (regent 1406-1418). In line with methods of historiographical analysis developed by Leonardo Funes, Georges Martin, and Aengus Ward, I combine close reading of the texts with consideration of the socio-political contexts of their production and reception. The first chapter examines narratives of Urraca's sole rule across four different centuries, from the contemporary Historia compostellana to the Crónicas anónimas de Sahagún, for which I propose a recontextualization as a wholly late-fifteenth century text. The diversity of these accounts underscores the flexible treatment of ruling queenship by medieval historians. The second chapter, closely tied to the first, considers representations of Berenguela in the Latin chronicles produced around the court of Fernando III and the vernacular chronicles of the Alfonsine tradition. The analysis of multiple contemporary accounts gives insight into the creation of competing narratives, and reveals the subtle transformations wrought upon the ultimately dominant version. The third chapter evaluates the singular image of María de Molina which emerges from the Crónica de los reyes de Castilla and chronicles of the reigns of Fernando IV and Alfonso XI. The fourth chapter explores the fragmented narratives of Catalina de Lancaster's regency which emerge from the vernacular, non-royalist histories of the reign of Juan II.