There has long been uncertainty about the relationship between the polities known as Strathclyde and Cumbria. Did medieval writers apply these terms to the same kingdom, or were Strathclyde and Cumbria separate entities? This debate has significant implications for our understanding of the politics of northern Britain during the period from the late ninth century to the twelfth. In this article I analyse the terminology in Latin, Old English, Old Norse, Welsh and Irish texts. I argue that Strathclyde developed into Cumbria: the expansion of the kingdom of Strathclyde beyond the limits of the Clyde valley necessitated the use of a new name. This process occurred during the early tenth century and created a Cumbrian kingdom that stretched from the Clyde to the south of the Solway Firth. The kingdom met its demise in the mid-eleventh century and Cumbrian terminology was subsequently appropriated for smaller ecclesiastical and administrative units. Yet these later usages should not be confused with the tenth-century kingdom, which encompassed a large area that straddled the modern Anglo-Scottish border.