My thesis offers a study of a collection of sermons for Sundays attributed to John Waldeby, an Austin Friar who was active in the city of York in the fourteenth century. By adopting the vertical approach of investigating a single medieval sermon collection in its entirety, it provides an insight into the preaching-support system of the mendicant friars and into different phases from the composition of the work to reception, in terms of both the use of the manuscripts by readers and the way the audience would have understood the sermons when they were preached. This thesis is divided into two major parts, each containing three chapters: the first part concerns the setting in which Waldeby's collection, the Novum opus dominicale, was written and used; the second part constitutes an analysis of the sermons in the work. Chapter 1 deals with the biography of Waldeby and sets out the basic information concerning the Novum opus dominicale. Chapter 2 identifies the proximate recipients of the work-the `youths' (iuuenes) assigned to Waldeby-and the wider lay audience, and examines various aspects of the training and education of friars within the Augustinian order. Through a close examination of the library catalogue of the York convent and the sources used in the Novum opus dominicale, Chapter 3 uncovers the existence of a reference book collection and describes the overall organisation of the library collection. Waldeby's collection emerges as the key text in the library, which was the material basis for the fundamental training of the friars and for supporting sermon composition. Chapter 4, the first chapter of Part II, extensively explores the form of Waldeby's sermons by a close comparative reading alongside Robert of Basevotn's Forma praedicandi. It reveals not only the intricacies of the techniques involved in the composition of `modern' sermons, but also elements of the mindset of preachers such as Waldeby. Chapter 5 focuses on the word `sign' (signum) which often appears in Waldeby's collection. The concept of sign plays a crucial role in both the mental framework through which he interpreted the Bible and the world, and the method by which he communicated his message to his audiences. The final chapter examines the overall design of the Novum opus dominicale in the broader perspective of de tempore collections. This study is accompanied by a critical transcription of one of the sermons of the Novum opus dominicale.