By 500 the Picts occupied the northern isles, and the north and the east as far south as Fife. Today their settlements can be generally identified by place names with a 'Pit' prefix, such as Pitlochry, and by the existence of carved symbol stones, like those found at Aberlemno in Angus. To the west, between Dumbarton and Carlisle, was a population of Britons. Many of the Briton leaders had Roman names, which suggests that they were a Romanized Celtic people, possibly a combination of tribes maintained by the Romans as a buffer between the Wall and the northern tribes, and peoples pushed west by the Anglo-Saxon invaders landing on the east coast. Both the Britons and the Picts spoke variations of P-Celtic, from which Welsh, Cornish and Breton developed.

On the west coast, to the north and west of the Britons, lived the Scotti, Irish-Celtic invaders who would eventually give their name to the whole country. The first Scotti arrived in the Western Isles from Ireland in the fourth century AD, and about a century later their great king, Fergus Mor, moved his base from Antrim to Dunadd, near Lochgilphead, where he founded the kingdom of Dalriada. The Scotti spoke Q-Celtic, the precursor of modern Gaelic. On the east coast, the Germanic Angles had sailed north along the coast to carve out an enclave around Dunbar in East Lothian. The final addition to the ethnic mix was also non-Celtic; from around 800 AD, Norse invaders began to arrive, settling mainly in the northern isles and the northeast of the mainland.

'the Picts traced succession through the female line'

The next few centuries saw almost constant warfare among the different groups. The main issue was land, but this was frequently complicated by the need of the warrior castes, who dominated all of these cultures, to exhibit martial prowess. Military conquests did play their part in bringing the peoples of Scotland together, but the most persuasive force was Christianity. Many of the Britons had been Christians since Roman times and it had been a Briton, St Ninian, who conducted the first missionary work among the Picts at the end of the fourth century. Attempts to convert the Picts were resumed in the sixth century by St Columba, who, as one of the Gaelic-speaking Scotti, demonstrated that Christianity could provide a bridge between the different tribes.

Christianity proved attractive to pagan kings because it seemed to offer them extra supernatural powers. As St Columba declared, when he inaugurated his cousin Aidan as king of Dalriada in 574: "Believe firmly, O Aidan, that none of your enemies will be able to resist you unless you first deal falsely against me and my successors". This combination of spiritual and political power, when taken with Columba's establishment of the island of Iona as a centre of Christian culture, opened the way for many peaceable contacts between the Picts and Scotti. Intermarriage became commonplace, and the Scotti king Kenneth MacAlpine, who united Dalriada and Pictland in 843, was the son of a Pictish princess (the Picts traced succession through the female line). Similarly, MacAlpine's creation of the united kingdom of Alba, later known as Scotia, was part of a process of integration rather than outright conquest. Kenneth and his successors gradually extended the frontiers of their kingdom by marriage and force of arms until, by 1034, almost all of what we now call Scotland was under their rule.