Welsh mythology is the mythology of the Welsh people. It consists partly of folk traditions developed in Wales, and partly of traditions developed by Britons elsewhere before the end of the first millennium. Some of this contains remnants of the mythology of pre-Christian Britain, surviving in much altered form in medieval Welsh manuscripts such as the Red Book of Hergest, the White Book of Rhydderch, the Book of Aneirin, and the Book of Taliesin.
The prose stories from the White and Red Books are known as the Mabinogion, a title given to them by their first translator, Lady Charlotte Guest, and also used by subsequent translators. Poems such as Cad Goddeu (The Battle of the Trees) and mnemonic list-texts like the Welsh Triads and the Thirteen Treasures of the Island of Britain, also contain mythological material. These texts also include the earliest forms of the Arthurian legend and the traditional history of post-Roman Britain.
Other sources include the 9th century Latin historical compilation Historia Britonum (the History of the Britons) and Geoffrey of Monmouth's 12th century Latin chronicle Historia Regum Britanniae (the History of the Kings of Britain), as well as later folklore, such as The Welsh Fairy Book by W. Jenkyn Thomas .
Welsh history, before the Romans, was learned orally by the Druids on fifteen year apprenticeships. Due to invasions by many, including the Romans and Saxons, history and mythology are confused due to the lack of written sources.