De Excidio et Conquestu Britanniae (Latin for "On the Ruin and Conquest of Britain") is a work by the 6th-century British cleric St 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 one of the most important sources for the history of Britain in the 5th and 6th centuries, as it is the only significant source for the period written by a near contemporary of the people and events described.