Traditionally, the medieval church has been divided between the 'Celtic' church, represented by Ireland, Scotland and, to a lesser extent, Wales, and the 'Roman' church of the continent and England. During the late eleventh and twelfth centuries, the structure of the 'Celtic' church was assimilated into the structure of the 'Roman' church through the activity of the Gregorian reform. This traditional picture has ignored the similarities between the church of the 'Celtic' lands and that of the continent. It has also not taken account of the interactions between the regions regarding ecclesiastical development. Once these similarities are considered, the impact of the Gregorian reform on the church in the Gaelic west appears similar to its impact on the rest of Christendom. The reform was not transformative in the sense of establishing something new, but it was transformative in creating unity in monasticism and stability in episcopal structure. The introduction and chapter one set the stage for the examination of the Gregorian reform's influence on Ireland and Scotland. The role and structure of the church on the continent and its tenth-century reforming movements are examined, as well as the aims of the Gregorian reformers. The church in Ireland and Scotland had similar roles and structures to the continent, including cenobitic and eremitic monasticism and an episcopal structure. Although the tenth-century continental reforms had no direct impact on Ireland and Scotland, the clergy in both regions were aware of current continental ecclesiastical trends. With the advent of the Gregorian reform, the church in Ireland and Scotland developed and solidified it s existing structures in accordance with many of the Gregorian principles. Chapters two and three demonstrate the successful introduction and tremendous growth of Gregorian monastic orders. While these new orders appeared new because they brought a unity of rule to Ireland and Scotland, they succeeded because of their similarities to traditional monastic practice in Ireland and Scotland. Chapters four and five chronicle the development of the existing episcopal structure in Ireland and Scotland. Both kingdoms had episcopal seats that maintained their status, and both countries had a traditional diocese that maintained it primacy. The reform did not introduce an episcopal structure, it simply encouraged royalty and clergy to define their episcopacy more precisely.