Welsh mythology, commonly known as Y Chwedlau (English: 'The Legends') consists of both folk traditions developed in Wales, and traditions developed by the Celtic Britons elsewhere before the end of the first millennium. As in most of the predominantly oral societies Celtic mythology and history were recorded orally by specialists such as druids (Welsh: derwyddon). This oral record has been lost or altered as a result of outside contact and invasion over the years. Much of this altered mythology and history is preserved in mediaeval Welsh manuscripts, which include the Red Book of Hergest, the White Book of Rhydderch, the Book of Aneirin and the Book of Taliesin. Other works connected to Welsh mythology include the ninth-century Latin historical compilation Historia Brittonum (History of the Britons) and Geoffrey of Monmouth's 12th-century Latin chronicle Historia Regum Britanniae (History of the Kings of Britain), as well as later Welsh folklore, such as the materials collected in The Welsh Fairy Book by William Jenkyn Thomas (1908).

As with other Insular Celts, no direct written accounts of the origins of the cosmos survive. We can assume that these Celts did have a complex cosmogony, given the accounts from classical authors about the depth of knowledge of the druids who passed down their knowledge via orature.

However, scholars can find connections to the broader Proto-Indo-European system in both the names of the deities themselves and in the later written tales that likely preserve remnants of the earlier orally transmitted narratives (see the "Pantheon" section below). Legends were not written down until after the Christianization of Britain, however, so these accounts are rather indirect; additionally, they likely evolved quite a bit over time anyway, as narrative systems typically do.

John T. Koch proposes that the name of the goddess Dôn, for instance, likely comes from *ghdhonos, meaning "the earth." In this sense she serves as the Welsh version of the *dheghom figure from Proto-Indo-European mythology, i.e. the primordial Earth Goddess from which all other gods originate. According to this theory, the Children of Dôn would be comparable to the Greek Titans.

John Carey suggests that the Fourth Branch of The Mabinogi, along with the Taliesin poems (especially Cad Goddeu), contain hints of the cyclicality of cosmogonic progressions.

Claude Sterckx proposes that Celtic mythology hints at a cyclical cosmogony given a key commonly repeated story pattern: a man (hero/king) and woman (earth or sovereignty goddess) face a rival for the woman's affection before uniting to birth a son who represents the life of the world; ultimately these figures die to create space for the cycle to repeat. For instance, the names of certain Welsh mythological figures point to this pattern, as John Koch suggests that Gwron ("divine husband") and Modron ("divine mother") united to create Mabon ("divine son"). This pattern occurs in The Mabinogion as well, with Pwyll and Rhiannon facing the rival Gwawl before uniting to conceive Pryderi. In both Mabon and Pryderi's cases, the divine son is taken by force soon after birth, with the divine parent(s) needing to wait years before ultimately reuniting with the son.

With this in mind, the transformations of various characters (especially Lleu and Taliesin) hint not just at reincarnation but perhaps even reconfigurations of the cosmos itself (along the lines of the Norse concept of Ragnarok as not merely signaling the end of existence, but heralding the beginning of a subsequent existence for the cosmos).

1) Elfydd: The Earth; the realm of humans