The legal underpinnings of the Western church experienced a major transformation during the twelfth and thirteenth centuries. This was a period in which papal legislation found its stride, in the form of conciliar decrees and papal decisions. Canon law collections provide a window into the rediscovery of Justinian's compilations, because they incorporated snippets of Roman law as it became available. Later papal legislation appears in other similar collections, including the Liber sextus of Pope Boniface VIII. Law professors at Bologna and elsewhere lectured on the collections of decretals, producing commentaries and summas. Medieval legal procedure relied heavily on both Roman and canon law. With respect to the laws of the church, the move towards complexity was also a product of the encounter with Justinian's Roman law. The thematic scope of canon law was in the main laid down with the Gratian's Decretum, which took its cue from the wide range of matters that French bishop Ivo treated in the Panormia.

Since the Carolingian age Catholic Christianity had spread from its heartland in the British Isles, France, the empire, Italy and northern Spain, to become the official religion of Bohemia, Poland and Hungary. A uniform though sparse church organisation existed throughout this huge area. The Christian west was divided into dioceses, though they varied considerably in size; but the provision of parishes was very uneven. All towns had at least one church while some had more than a hundred, but even in parts of the west which had been Christian for centuries many rural areas, were still served by the clergy of a central minster. The growth of parishes increased the possibility of conflict between clergy and laity. The cost of maintaining churches and priests was largely met from tithes, theoretically levied on all sources of income, but normally on the principal grain crops, but tithes were frequently impropriated by lay patrons, and this was a source of litigation.

This chapter concentrates on the history of the western church in the eighth and ninth centuries, which have generally been recognised as pivotal in the development of ecclesiastical organisation, canon law and the liturgy. In the middle of the ninth century the Notitia Galliarum was used for polemical purposes by the compilers of the Pseudo-Isidorian Decretals to assert the primacy of one see over another, and more specifically, one metropolitan over another. The territorial structure of the churches in lands where Celtic Christians lived has always been an enigma in that Roman territorial structures had not existed in these areas. Visigothic Spain had an ecclesiastical organisation highly peculiar to that church. By the end of the ninth century liturgical rites in Western Europe, whether daily or occasional, were perhaps even more varied and rich than they had been at the beginning of the eighth century.