One of the first historical references to Saxons occurs in a description of a ‘barbarian conspiracy’, consisting of Picts, Scots and Saxons, attacking Roman Britain ( Yorkshire ) in 367 AD. It is possible that the number of Saxon invaders who settled in Yorkshire had been augmented by Germanic mercenaries who had originally been employed by the Romans to defend their Northern frontier from these very same Picts and North Britons. In any event, by the sixth century Anglo-Saxon colonies on the east coast of Britain had been organised into two separate kingdoms of Bernicia (north of the Tyne ) and Deira (south of the Tyne ). By the mid-seventh century the Anglo-Saxon kings of Northumbria ( Bernicia and Deira) had established their overlordship in the North-British kingdoms of Rheged and Gododdin. During this period the Northumbrians also came into conflict with a king of Dál Riata, Aedán mac Gabráin, at Degsastan . The site of this battle has not yet been identified.
Before 685 AD it is clear that Northumbrian overlordship was firmly established in the area of central Scotland known as Manau.This name is preserved in at least two place-names, Slamannan and Clackmannan, on either side of the Forth estuary, and a Northumbrian bishopric was established at Abercorn under bishop Trumwine. A number of early Anglo-Saxon place names in Lothian, including Whittinghame, Coldingham and Haddington, probably date from this period. However, in 685 AD king Ecgfrith of Northumbria was defeated by Bruide son of Bili, king of the Pictish province of Fortriu , at Nechtanesmere near Forfar. According to one early eighth-century English source (Bede), the Picts, Scots and some North Britons recovered their freedom as a result of this victory. Eventually, during the reign of Causantín mac á eda (Constantine II: 900 to c.942 AD), king of Alba, the rulers of Northumbria accepted Scottish overlordship and the entire territory of Lothian was annexed to Scotland when Máel Coluim mac Cináeda (Malcolm II) won the battle of Carham in 1018 AD.
The language the Northumbrian Anglo-Saxons spoke was called Inglis . After 1018 AD, Inglis (which became known as Scottis (Scots) during the fifteenth century) began to spread into other areas of Scotland , particularly in burghs where it became the favoured language of trade. For example, it is likely that the burgh of Aberdeen was founded and settled by people of Anglo-Saxon or Flemish origin and Eadward (an Anglo-Saxon) became bishop of Aberdeen before 1150 AD, just after the Gaelic Notes had been inserted into the Book of Deer. Of course the vast majority of the people in north-east Scotland during the twelfth century would have spoken Gaelic; the trading burghs would have been considered large if they had a population of a few hundred people. However, Buchan seems to have shifted more towards the Inglis language during the fourteenth century. One possible reason behind this shift in language may have been the actions of Robert I. After 1306, when he seized the throne from the rightful king of Scots, John I Balliol, and murdered his main political rival, John Comyn of Badenoch, he systematically dismembered the Comyn earldom of Buchan. Thereafter, he granted a large number of lands in Buchan to families from southern Scotland , like Gordon, Hay, Douglas and Keith, whose first language would probably not have been Gaelic. The use of Gaelic has been retreating westwards from the North-East ever since.