As the only daughter of Blanche of Castile, one of France's most powerful queens, and as the sister of the Capetian saint Louis IX, Isabelle of France (1225-1270) was situated at the nexus of sanctity and power during a significant era of French culture and medieval history. In this ground-breaking examination of Isabelle's career, Sean Field uses a wealth of previously unstudied material to address significant issues in medieval religious history, including the possibilities for women's religious authority, the creation and impact of royal sanctity, and the relationship between men and women within the mendicant orders. Field reinterprets Isabelle's career as a Capetian princess. Isabelle was remarkable for choosing a life of holy virginity and for founding and co-authoring a rule for the Franciscan abbey of Longchamp. Isabelle did not become a nun there, but remained a powerful lay patron, living in a modest residence on the abbey grounds. Field maintains that Isabelle was a key actor in creating the aura of sanctity that surrounded the French royal family in the thirteenth century, underscoring the link between the growth of Capetian prestige and power and the idea of a divinely ordained, virtuous, and holy royal family. Her contemporary reputation for sanctity emerges from a careful analysis of the Life of Isabelle of France written by the third abbess of Longchamp, Agnes of Harcourt, and from papal bulls, letters, and other contemporary sources that have only recently come to light. Isabelle of France explores the life of a remarkable figure in French and Franciscan history.