The name Rheged appears regularly as an epithet of a certain Urien in a number of early Welsh poems and royal genealogies. His victories over the Anglian chieftains of Bernicia in the second half of the 6th century are recorded by Nennius and celebrated by the bard Taliesin, who calls him "Ruler of Rheged". He is thus placed squarely in the North of Britain and perhaps specifically in Westmorland when referred to as "Ruler of Llwyfenydd" (identified with the Lyvennet Valley). Later legend associates Urien with the city of Carlisle (the Roman Luguvalium), only twenty-five miles away; Higham suggests that Rheged was "broadly conterminous with the earlier Civitas Carvetiorum, the Roman administrative unit based on Carlisle". Although it is possible that Rheged was merely a stronghold, it was not uncommon for sub-Roman monarchs to use their kingdom's name as an epithet. It is generally accepted, therefore, that Rheged was a kingdom covering a large part of modern Cumbria.
Place-name evidence, e.g., Dunragit (possibly "Fort of Rheged") suggests that, at least in one period of its history, Rheged lay in Dumfries and Galloway. More to the point, recent archaeological excavations at Trusty's Hill, a vitrified fort near Gatehouse of Fleet, and the analysis of its artifacts in the context of other sites and their artifacts have led to claims that the kingdom was centred on Galloway early in the 7th century.
More problematic interpretations suggest that it could also have reached as far south as Rochdale in Greater Manchester, recorded in the Domesday Book as Recedham. The River Roch on which Rochdale stands was recorded in the 13th century as Rached or Rachet. These place-names may (apparently) incorporate the element 'Rheged' precisely because they lay on or near its borders. Certainly Urien's kingdom stretched eastward at one time, as he was also "Ruler of Catraeth" (Catterick in North Yorkshire).
The traditional royal genealogy of Urien and his successors traces their ancestry back to Coel Hen (considered by some to be the origins of the Old King Cole of folk tradition), who is considered by many to be a mythical figure; if he has some historicity, he may have ruled a considerable part of the North in the early 5th century. All of those listed below may have ruled in Rheged, but only three of their number can be verified from external sources:
A second royal genealogy exists for a line, perhaps of kings, descended from Cynfarch Oer's brother: Elidir Lydanwyn. According to Bonedd Gwŷr y Gogledd Elidir's son, Llywarch Hen, was a ruler in North Britain in the 6th century. He was driven from his territory by princely in-fighting after Urien's death and was perhaps in old age associated with Powys. However, it is possible, because of internal inconsistencies, that the poetry connected to Powys was associated with Llywarch's name at a later, probably 9th century, date. Llywarch is referred to in some poems as king of South Rheged, and in others as king of Argoed, suggesting that the two regions were the same. Searching for Llywarch's kingdom has led some historians to propose that Rheged may have been divided between sons, resulting in northern and southern successor states. The connections of the family of Llywarch and Urien with Powys has suggested to some, on grounds of proximity, that the area of modern Lancashire may have been their original home.