This dissertation examines how the religious thinking of Prosper Guéranger (1805-1875) developed during the course of his life and how it was distinct from more mainstream strands in Ultramontane piety. Although influenced as a young man by the works of Félicité de Lamemlais, he was particularly attracted by the latter's writing 011 ecclesiastical history and by his critique of the Concordat agreement of 1801 which curtailed the independence of the clergy. III at ease in the moral and liturgical climate of the Gallican Church, he established a monastic community at Solesmes in the Sarthe valley in 1833 and, against all odds, he pioneered liturgical reform and wrote and published prolifically from this base all his life. His early inspiration came from his reading of the Fathers of the Church and from a conviction that Christian doctrine was founded in Trinitarian theology. This led him to attribute a key role to Mary because of the part she played in the Mystery of the Incarnation. After 1850 he explored how this message had been carried forward by the saints and mystics whose writings were embedded in liturgical practice, at least until the end of the medieval period. Although Guéranger was not a conventionally Romantic writer, his work has features in common with the more famous historiographers and history painters, who were working in the 1830s and 1850s. He shared with them a conunitment to understand the past in order to improve the future and he read widely and engaged in debate with those writers concerned to discover the historical rather than the doctrinal Jesus. His monastic vocation was central to his thinking and his experiences as abbot and pastor gave him an insight into the emotional and spiritual lives of the local Catholic bourgeoisie. A lateral thinker, he failed to convince his more positivist peers but his insights into the religious mind, allied with his conunitment to the idea of monastic practice as exemplary, allowed him to offer practical solutions to religious 'l~du-'ference and to engender conunitment to faith through a better understanding of the purposes of liturgy.