De Excidio et Conquestu Britanniae (English: On the Ruin and Conquest of Britain) is a work written in Latin in the late fifth or sixth century by the British religious polemicist Gildas. It is a sermon in three parts condemning the acts of Gildas' contemporaries, both secular and religious, whom he blames for the dire state of affairs in sub-Roman Britain. It is widely acknowledged by historians that the text cannot be straightforwardly used to construct a reliable narrative history of fifth- and sixth-century Britain. It nevertheless is one of the most important sources for the history of Britain in the fifth and sixth centuries, as it is the only significant historical source for the period written by a near contemporary of the people and events described.
Part I contains a narrative of British history from the Roman conquest to Gildas' time; it includes references to Ambrosius Aurelianus and the Britons' victory against the Saxons at the Battle of Mons Badonicus. Part II is a condemnation of five kings for their various sins, including both obscure figures and relatively well-documented ones such as Maelgwn Gwynedd. Part III is a similar attack upon the British clergy of the age.