The settlement of Great Britain by Germanic peoples from continental Europe led to the development of an Anglo-Saxon cultural identity and a shared Germanic language—Old English—whose closest known relative is Old Frisian, spoken on the other side of the North Sea. The first Germanic speakers to settle in Britain permanently are likely to have been soldiers recruited by the Roman administration in the 4th century AD, or even earlier. In the early 5th century, during the end of Roman rule in Britain and the breakdown of the Roman economy, larger numbers arrived, and their impact upon local culture and politics increased.

There is ongoing debate about the scale, timing and nature of the Anglo-Saxon settlements and also about what happened to the existing populations of the regions where the migrants settled. The available evidence includes a small number of medieval texts which emphasize Saxon settlement and violence in the 5th century but do not give many clear or reliable details. Linguistic, archaeological and genetic information have played an increasing role in attempts to better understand what happened. The British Celtic and Latin languages spoken in Britain before Germanic speakers migrated there had very little impact on Old English vocabulary. According to many scholars, this suggests that a large number of Germanic speakers became important relatively suddenly. On the basis of such evidence it has even been argued that large parts of what is now England were clear of prior inhabitants, perhaps due to mass deaths from the Plague of Justinian. However, a contrasting view that gained support in the late 20th century suggests that the migration involved relatively few individuals, possibly centred on a warrior elite, who popularized a non-Roman identity after the downfall of Roman institutions. This hypothesis suggests a large-scale acculturation of natives to the incomers' language and material culture. In support of this, archaeologists have found that, despite evidence of violent disruption, settlement patterns and land use show many continuities with the Romano-British past, despite profound changes in material culture.

A major genetic study in 2022 which used DNA samples from different periods and regions demonstrated that there was significant immigration from the area in or near what is now northwestern Germany, and also that these immigrants intermarried with local Britons. This evidence supports a theory of large-scale migration of both men and women, beginning in the Roman period and continuing until the 8th century. At the same time, the findings of the same study support theories of rapid acculturation, with early medieval individuals of both local, migrant and mixed ancestry being buried near each other in the same new ways. This evidence also indicates that in the early medieval period, and continuing into the modern period, there were large regional variations, with the genetic impact of immigration highest in the east and declining towards the west.

One of the few written accounts of the period is by Gildas, who probably wrote in the early 6th century. His account influenced later works which became more elaborate and detailed but which cannot be relied upon for this early period. Gildas reports that a major conflict was triggered some generations before him, after a group of foreign Saxons was invited to settle in Britain by the Roman leadership in return for defending against raids from the Picts and Scots. These Saxons came into conflict with the local authorities and ransacked the countryside. Gildas reports that after a long war, the Romans recovered control. Peace was restored, but Britain was weaker, being fractured by internal conflict between small kingdoms ruled by "tyrants". Gildas states that there was no further conflict against foreigners in the generations after this specific conflict. No other local written records survive until much later. By the time of Bede, more than a century after Gildas, Anglo-Saxon kingdoms had come to dominate most of what is now modern England. Many modern historians believe that the development of Anglo-Saxon culture and identity, and even its kingdoms, involved local British people and kingdoms as well as Germanic immigrants.

A traditional account of Anglo-Saxon immigration has been influential since at least the 8th century, when Bede outlined his reconstruction of what had happened some centuries earlier. While he partly based upon his work upon earlier records such as the near contemporary Gildas, these gave a very incomplete picture, and he added many details. Modern scholars see several aspects of his expanded account as questionable, while popular and fictional accounts, including even Arthurian legend, have tended to take it for granted.

In the traditional account, there was a single large coordinated invasion of Anglo-Saxons into Britain after the end of Roman rule in 411. This adventus saxonum represented the main immigration event, and this was followed by a period where small, pagan Anglo-Saxon kingdoms in the east fought small Celtic Christian kingdoms in the west, and bit by bit the Anglo-Saxons defeated the Britons and took over the country, and in this way England became English by force. In this traditional account ethnic Anglo-Saxons and ethnic Britons were from the beginning distinct and separated peoples, conscious of the war between their nations. It was envisioned that Britons living in Anglo-Saxon kingdoms either had to move or convert to a foreign culture.

In contrast, modern scholars generally believe that Germanic speakers started arriving in Britain before the end of Roman rule, probably mainly as soldiers. They may have formed a significant part of Romano-British society at the end of Roman rule, and their culture probably continued to be especially associated with the military. That immigration and conflict involving Germanic speakers increased during the 5th century, after the end of Roman rule, is still widely accepted by scholars, but it is no longer assumed that this necessarily involved the immediate formation of small Anglo-Saxon kingdoms, or a straightforward conflict between two opposed ethnic groups. Although such ethnic kingdoms were known to Bede from his own time, much uncertainty remains about the way in which these kingdoms developed between the time of Gildas and the time of Bede.

The area of present-day England was part of the Roman province of Britannia from 43 AD. The province seems unlikely ever to have been as deeply integrated into Roman culture as nearby Continental provinces, however, and from the crisis of the third century Britain was often ruled by Roman usurpers who were in conflict with the central government in Rome, such as Postumus (about 260–269), Carausius (286–293), Magnentius (350–353), Magnus Maximus (383–388), and Constantine III (407–411).