This book examines the religious authority of St Birgitta of Sweden, the charismatic moral reformer and controversial female visionary of the fourteenth century, emphasising both representations of her prophetic mission and debates about her authenticity as a medium of divine revelation. It illuminates Birgitta's view of herself as a prophet of moral reform by explaining how her Revelations depict her religious mission and place in salvation history, going on to reconstruct interactions between Birgitta and her contemporaries, including the significance of her prophetic authority vis-a-vis the priestly authority of her male clerical associates. Finally, it analyses arguments about women's suitability for mediating the divine word in posthumous attacks and defences of her claims to prophesy. Through a close examination of Birgitta's lengthy Revelations, canonization documents, and texts by her posthumous defenders and detractors, this study demonstrates that members of her audience perceived her to be both a vibrant source of supernatural power and a dangerous transgressor of conventional boundaries. Informed by sociological studies of prophetic authority, it contributes to our knowledge of Birgitta herself as well as to our understanding of the dynamics of women's spiritual authority. Professor CLAIRE SAHLIN teaches at Texas Woman's University.