Gildas — also known as Gildas Badonicus, Gildas fab Caw and Gildas Sapiens — was a 6th-century British monk best known for his scathing religious polemic De Excidio et Conquestu Britanniae, which recounts the history of the Britons before and during the coming of the Saxons. He is one of the best-documented figures of the Christian church in the British Isles during the sub-Roman period, and was renowned for his Biblical knowledge and literary style. In his later life, he emigrated to Brittany where he founded a monastery known as Saint-Gildas-de-Rhuys.

The settlement of Great Britain by diverse Germanic peoples, who eventually developed a common cultural identity as Anglo-Saxons, changed the language and culture of most of what became England from Romano-British to Germanic. This process principally occurred from the mid-fifth to early seventh centuries, following the end of Roman rule in Britain around the year 410. The settlement was followed by the establishment of the Heptarchy, Anglo-Saxon kingdoms in the south and east of Britain, later followed by the rest of modern England, and the south-east of modern Scotland. The exact nature of this change is a topic of on-going research. Questions remain about the scale, timing and nature of the settlements, and also about what happened to the previous residents of what is now England.