The Anglo-Saxon settlement of Britain was the invasion and migration of Germanic peoples from continental Europe to Great Britain during the Early Middle Ages, specifically the arrival of the Anglo-Saxons in Britain after the demise of Roman rule in the 5th century.

The stimulus, progression and impact of the Germanic settlement of Britain is subject to considerable disagreement, prompted by varying accounts and evidence. The traditional division of the migrants into Angles, Saxons and Jutes — peoples from Angeln, Saxony and Jutland — comes from the Historia ecclesiastica gentis Anglorum, an 8th-century Latin history written by the Venerable Bede about Christianity in the Kingdom of England. Historical and archaeological research in the early 20th century has shown that a wider range of Germanic peoples from the coasts of Frisia, Lower Saxony, and Sweden also moved to Britain in this era. The Anglo-Saxons supplanted Celtic culture and society in much of southern and central Britain and contributed to the creation of Anglo-Saxon England and the use of the Old English language.

The term Anglo-Saxon is used by some historians to designate the Germanic tribes who invaded and settled the south and east of Britain beginning in the early 5th century and the period from their creation of the English nation up to the Norman conquest. The Anglo-Saxon era denotes the period of English history between about 550 and 1066. The term is also used for the language, now known as Old English, that was spoken and written by the Anglo-Saxons and their descendants in England (and part of southeastern Scotland) between at least the mid-5th century and the mid-12th century.

The Benedictine monk Bede, writing in the early 8th century, identified the English as the descendants of three Germanic tribes:

Their language, Old English, which derived from Ingvaeonic West Germanic dialects, transformed into Middle English from the 11th century. Old English was divided into four main dialects: West Saxon, Mercian, Northumbrian and Kentish.

The term Anglo-Saxon can be found in documents produced in the time of Alfred the Great, who seems to have frequently used the titles rex Anglorum Saxonum and rex Angul-Saxonum (king of the English Saxons). The Old English terms ænglisc ('Angle-kin') and Angelcynn ('gens Anglorum') had already lost their original sense of referring to the Angles, as distinct from the Saxons, when they are first attested. In their earliest sense they referred to the nation of Germanic peoples who settled eastern Britain from the 5th century.^{[citation needed]} The indigenous Britons, who wrote in both Latin and Welsh, referred to these invaders as 'Saxones' or 'Saeson' – the word Saeson is the modern Welsh word for 'English people'; the equivalent word in Scottish Gaelic is Sasannach and in the Irish language, Sasanach.

King Arthur is a legendary British leader of the late 5th and early 6th centuries, who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the early 6th century. The details of Arthur's story are mainly composed of folklore and literary invention, and his historical existence is debated and disputed by modern historians. The sparse historical background of Arthur is gleaned from various sources, including the Annales Cambriae, the Historia Brittonum, and the writings of Gildas. Arthur's name also occurs in early poetic sources such as Y Gododdin.

The legendary Arthur developed as a figure of international interest largely through the popularity of Geoffrey of Monmouth's fanciful and imaginative 12th-century Historia Regum Britanniae (History of the Kings of Britain). Some Welsh and Breton tales and poems relating the story of Arthur date from earlier than this work; in these works, Arthur appears either as a great warrior defending Britain from human and supernatural enemies or as a magical figure of folklore, sometimes associated with the Welsh Otherworld, Annwn. How much of Geoffrey's Historia (completed in 1138) was adapted from such earlier sources, rather than invented by Geoffrey himself, is unknown.