The Litlyngton Missal, Westminster Abbey Library MS 37, is a lavishly illuminated English service book commissioned by Abbot Nicholas Litlyngton 1383-4 and donated to his Benedictine monastery at Westminster. This thesis examines the life of this medieval ecclesiastical patron and investigates how his missal is an expression not simply of a desire to be commemorated, but is also a reflection of his priorities as a member of Westminster’s monastic community. While the study’s emphasis is on the missal’s iconography, both text and image are contextually examined in order to better appreciate the patron’s intended messages of personal devotion to the cult of the Blessed Virgin Mary, the abbey’s promotion, and protection of its privileges. This study scrutinizes the abbey’s particular status in relation to the crown and how this is reflected through the missal, most especially through the inclusion of coronation orders and royal exequies. Considering the rubrics and illuminations of these ceremonies through the lens of Westminster Abbey and its abbot elucidates their authorship and clarifies why, atypically, they were included in a service book of this kind. Analysis of documentation and examination of the book’s stages of creation affords a better understanding of the missal’s production than has been obtained to date and shows that there is an overarching aesthetic cohesion to the book. The thesis offers a critical reappraisal of the missal’s illumination and reveals previously unacknowledged innovation and subtlety. The thesis considers what images occur, where, and how they relate to the text. The findings regarding the imagery are contextualised by comparison with illumination schemes of other English missals of fourteenth and fifteenth century missals and service books. The thesis discussion begins with a biographical study of Nicholas Litlyngton in chapter one, providing a clear context to the man who commissioned the missal. Chapter two considers Litlyngton specifically in his role as patron of the missal. The focus of chapter three is the production of the missal, focusing on its scribe, the illuminators, and their style. Discussion of the contested matter of number of artists and attribution of work also occurs in this chapter. Chapter four scrutinises the text and images connected to the royal ceremonies and examines the motivation behind their inclusion in the missal. The final chapter considers the manuscript’s iconographic programme through a comparative study of other English missals, and interprets the extent of convention or innovation in the Litlyngton Missal’s illuminations. Chapter five also examines messages contained in the images and reflects on their significance and purpose.