The OPVS project challenges previous studies by resorting to the medieval readers perspective in order to rank medieval Europe s bestselling vernacular religious texts: we shall focus on successful texts with voluminous manuscript traditions, taking into account the most popular ones, extant in over 60 copies. All were written (or translated) in French, English or German: texts composed in other vernacular languages were not so widely disseminated.These manuscripts will bear testimony as to how, when and where they were produced. Because they have traces of their owners and have been annotated by medieval readers, they can also yield new data on medieval pious practices and religious reading.Previous studies have generally eschewed this documentary evidence, except in the case of deluxe copies. This neglect is due both to a greater interest for Latin treatises with lofty ambitions, and to the impossible challenge, for a single researcher, of dealing with multiple manuscripts, kept in libraries worldwide hence the need for new investigating methods and teamwork.Our project s novel perspective consists in studying these manuscripts both individually and comparatively, combining a traditional erudite approach and new serial methods (quantitative codicology). Relying on an innovative database, our aim is a) to develop a sophisticated typology of medieval religious manuscripts in vernacular languages, b) to determine when, where and in which milieus the selected texts were copied and circulated, giving new insights into all the agents involved in the European book trade of religious literature in vernacular languages especially workshops and book owners, and finally c) to produce comparative studies of medieval literacy and readership in France, England and Germany.