By the later Middle Ages Geoffrey of Monmouth’s account of the British past was the most important text of Welsh history, both in Latin and the vernacular, and had furthermore a political significance during events such as the Glyn Dŵr revolt. Geoffrey wrote within a Welsh context, with known Welsh sources as well as a broader understanding of fundamental Welsh historical ideas. Geoffrey’s place within existing ideas of the Welsh past is discussed, followed by an assessment of the evidence for responses to and uses of Geoffrey’s work in the 12th, 13th and 14th centuries. The Latinate response is traceable through surviving manuscripts and the works of Gerald of Wales and Madog of Edeirnion, whereas vernacular uses of Galfridian ideas include numerous Welsh translations and references in court poetry. Overall a dialogue between Geoffrey’s work and other ideas of the Welsh past is frequently discernible through the adaptation of Geoffrey’s work and of native texts, such as Trioedd Ynys Prydein, in response to each other. The full acceptance of the narrative of De gestis Britonum as part of a foundational narrative of Welsh history is best appreciated through the vernacular “Welsh Historical Continuum” which was developed by the mid-14th century, and in such a guise its importance continued into the modern period.