This thesis arose from a need for a re-evaluation of opus Anglicanum, a somewhat discounted art form which was nevertheless central to the cultural output of medieval England. It is concerned with looking closely at a couple of important aspects. First, the available evidence is considered, with a view to exploring whether long-held assumptions about the subject can actually be substantiated; second, a detailed study of iconography is made, in an attempt to find an explanation for particular choices. Among the extant English medieval ecclesiastical embroideries the great copes, covering the period from c1270 to c1330, offer the most fruitful opportunities for study. Thus, the focus is on these for general concerns and for more particular issues four "narrative" copes have been examined in detail. Early assessment of the gamut of imagery disclosed certain striking features--the individuality and doctrinal exactitude of the various iconographic programmes, the singular absence of some central theological themes and the ubiquitous nature of the angelic presence among the representations--which indicated lines of enquiry and determined the parameters of study. In the course of laying out the evidence such primary sources as there are, are reviewed and assumptions regarding possible workshop practices and issues of patronage are examined. On the technical side, the manufacture of these precious embroideries is explored and the vexed question of who was responsible for the designs is considered. The findings reveal that, contrary to widely held opinion, the luxury copes were liturgical vestments, with a crucial role to play both within the service and the meaning of the High Mass itself The cherished belief that the twenty processional vestments which are known today represent a mere fraction of the original output is challenged and a diametrically opposed view is put forward - that what there is, is the greatest part of what there was.