Following the collapse of the Roman occupation, around AD 410, Britain fragments into a series of kingdoms, and local tribal leaders begin to re-emerge to assume the mantle of the Roman governors.

There are considerable movements of peoples; Anglo-Saxon migrations into the eastern parts of Britain, Irish crossings to Scotland, Wales and Cornwall, and a British exodus to NW France – which came to be known as Little Britain, then Brittany.

Imported pottery, amphorae and fine wares, found at Tintagel (and in smaller numbers at a few other sites), indicate continuing cultural and economic links with the Mediterranean. This may have been a factor in the arrival of Christianity in the 5th and 6th centuries, though missionaries from Ireland seem to have played the major role. Religious communities are established in enclosed sites known as lanns and many commemorative crosses and inscribed stones are set up across the countryside. Christianity is a major force for social change throughout this period.
 
Egbert, king of Wessex, defeats a combined Cornish and Danish army at Hingston Down in AD 838, leaving Cornwall a vassal kingdom.
 
Many settlements with names beginning with the prefixes 'Tre' or 'Bod' are thought to have their origins in the period from the 7th century up to the Norman conquest, by which time the countryside is thickly populated.