The historical basis for the King Arthur legend has long been debated by scholars. One school of thought, citing entries in the History of the Britons and Welsh Annals, sees Arthur as a genuine historical figure, a Romano-British leader who fought against the invading Anglo-Saxons sometime in the late 5th to early 6th century. The other text that seems to support the case for Arthur's historical existence is the 10th-century Annales Cambriae. The latest research shows that the Annales Cambriae was based on a chronicle begun in the late 8th century in Wales. Additionally, the complex textual history of the Annales Cambriae precludes any certainty that the Arthurian annals were added to it even that early. They were more likely added at some point in the 10th century and may never have existed in any earlier set of annals.

This lack of convincing early evidence is the reason many recent historians exclude Arthur from their accounts of post-Roman Britain. In the view of historian Thomas Charles-Edwards there may well have been an historical Arthur, but that a historian can as yet say nothing of value about him. These modern admissions of ignorance are a relatively recent trend; earlier generations of historians were less skeptical. Historian John Morris made the putative reign of Arthur the organizing principle of his history of post-Roman Britain and Ireland. Even so, he found little to say about a historical Arthur. Partly in reaction to such theories, another school of thought emerged which argued that Arthur had no historical existence at all. Morris's Age of Arthur prompted archaeologist Nowell Myres to observe that no figure on the borderline of history and mythology has wasted more of the historian's time. Arthur is not mentioned in the Anglo-Saxon Chronicle or named in any surviving manuscript written between 400 and 820. He is absent from Bede's early-8th-century Ecclesiastical History of the English People, another major early source for post-Roman history.

Some scholars argue that Arthur was originally a fictional hero of folklore-or even a half-forgotten Celtic deity-who became credited with real deeds in the distant past. They cite parallels with figures such as the Kentish totemic horse-gods Hengest and Horsa, who later became historicized. Bede ascribed to these legendary figures a historical role in the 5th-century Anglo-Saxon conquest of eastern Britain.

Historical documents for the post-Roman period are scarce. Of the many post-Roman archeological sites and places, only a handful have been identified as "Arthurian," and these date from the 12th century or later. Archaeology can confidently reveal names only through inscriptions found in reliably dated sites. In the absence of new compelling information about post-Roman England, a definitive answer to the question of Arthur's historical existence is unlikely.