In and around the 870s, Britain was transformed dramatically by the campaigns and settlements of the Great Army and its allies. Some pre-existing political communities suffered less than others, and in hindsight the process helped Scotland and England achieve their later positions. By the twelfth century, the rulers of these countries had partitioned the former kingdom of Northumbria. This thesis is about what happened in the intervening period, the fate of Northumbria's political structures, and how the settlement that defined Britain for the remainder of the Middle Ages came about. Modern reconstructions of the era have tended to be limited in scope and based on unreliable post-1100 sources. The aim is to use contemporary material to overcome such limitations, and reach positive conclusions that will make more sense of the evidence and make the region easier to understand for a wider audience, particularly in regard to its shadowy polities and ecclesiastical structures. After an overview of the most important evidence, two chapters will review Northumbria's alleged dissolution, testing existing historiographic beliefs (based largely on Anglo-Norman-era evidence) about the fate of the monarchy, political community, and episcopate. The impact and nature of ‘Southenglish' hegemony on the region's political communities will be the focus of the fourth chapter, while the fifth will look at evidence for the expansion of Scottish political power. The sixth chapter will try to draw positive conclusions about the episcopate, leaving the final chapter to look in more detail at the institutions that produced the final settlement.