Durham Cathedral is known to possess one of the largest and most intact collections of 17th-century liturgical music manuscripts in the world. That so much material survived the trauma of the Commonwealth is fortuitous indeed. The history of the pre-Civil War manuscripts has already been researched, and those after the Restoration have been investigated to a degree. The present research is concerned with a detailed study of the music composed by the many Durham musicians of the 17th century contained in the manuscripts, and their related sources. In total over 80 works by 20 composers are represented in varying degrees of completeness. These range from complete autograph texts through to solitary tenor parts. The study is concerned solely with the scene at Durham. To enlarge on earlier research, a detailed study of the manuscripts from the second half of the century is presented. These show the stability of the repertoire and the introduction of much new material towards the end of the century. A newlycompiled catalogue of the related manuscripts at Peterhouse, Cambridge is presented as an appendix. A representation of every piece of Durham-composed music is given. Extracts only are presented of fragmentary items, and also for reasons of space and time where a whole piece of music does not reveal anything significant. Reconstructions are presented of works with one or two parts missing, or where a large amount of material can be recovered from an extant organ part. Transcriptions are presented in cases where a complete text survives. The study is divided into two volumes. Volume one describes the music and its sources, and volume two contains musical transcriptions.