How did Isabel the princess become Isabel the queen? I mean this not in the legal sense of “how” – that is, the problem of her inheritance of the Kingdom of Castile from her half brother Enrique and her successful fight to keep it, which has been often and well studied. What concerns me here is the more complex “become,” meaning the transformation of a young princess relatively unschooled in the arts of governance into a queen who ruled in her own right. When considering the process of becoming a queen, some scholars have focused on the impact that her mother, Isabel of Portugal, had on her character and religiosity (Liss 2004b: 11–25, 53–72; Val Valdivieso 2003: 41–44). Others have turned to treatises on rulership written by her advisers (both clerical and lay), the books she possessed, and the literature she may have read (Lehfeldt 2000: 35–44; Weissberger 2004: 28–44). Their insights have illuminated vividly the forces that shaped the political culture of this most famous of Spanish queens and her distinctive practice of queenship. For most queens in any age, whose political power is limited, this would suffice. but to understand how Isabel became a queen with full regal power in Castile but whose authority was limited in Aragon by Fernando's, it is necessary to look at what it meant to be an Aragonese queen; for that, we can turn to Fernando's mother, Juana Enríquez. Although Isabel is the queen most readers will recognize, my argument will shift the emphasis to the less familiar Queen Juana.