This dissertation explores the doctrinal contribution of St. Catherine of Siena to the mystical literary tradition. St. Catherine of Siena's three works: Le Orazioni, II Dialogo della divina provvidenza and the Epistolario have been studied as individual texts, yet an extensive collective examination of these works remained absent. My project explores the spiritual purpose of St. Catherine of Siena's Dialogo della divina provvidenza. Through a study of the Epistolario and Orazioni as secondary works, I investigate how these texts add to the Dialogo, the greater doctrinal masterwork. Chapter 1 discusses the importance of St. Catherine of Siena's literary portrayal as depicted through the eyes of her hagiographers. I argue that the unity and uniqueness of the hagiographies of St. Catherine are based upon three criteria: the authors' borrowing of narratives within the hagiographies, the repeated references to Raymond of Capua as Catherine's most reliable biographer, and allusions to Catherine's theoretical prominence. Chapter 2 explores St. Catherine's spiritual formation and considers the impact of Dominican preaching, her confessors, and the written works of Domenico Cavalca. In Chapter 3, I demonstrate how materials developed in the Epistolario and Orazioni are eventually defined and refined in the Dialogo. Chapter 4 considers the lasting impact of St. Catherine's literary work on 15^th-16^th century Dominican women mystics. Stefana Quinzani (1457-1530), Caterina de' Ricci (1522-1590) and Lucia Broccadelli da Narni (1476-1544) produced texts that reflect the terminology found in Catherine of Siena's Dialogo. The Dominican tertiaries' incorporation and manipulation of Catherinian dogmatic concepts accentuate St. Catherine's role as a female pioneer in mystical and literary thought.