The extent to which southern kings of England, from the tenth century onwards, were able to claim authority over the kingdom of Northumbria, is a question of considerable importance in any consideration of the unification of England in the Anglo-Saxon period. Scholars have previously made use of a range of historical evidence in the pursuit of answers, including the testimony of, for example, narrative texts, coins and place-names. But the royal charters and diplomas of Anglo-Saxon Northumbria have never before been harnessed in such discussions and this article examines what they reveal about structures of power within England and likewise within Northumbria itself.