Classical commentators writing about the Celts during the first century BC and the first few centuries AD, initially mention the Druids as prominent members of Celtic society. They are discussed by several writers, some of whom - such as Caesar (Gallic Wars), Strabo (Geography) and Pliny (Natural History) - speak at great length about the various functions of the Druids, while others make only brief allusions to them. The second body of primary literary evidence concerning the Druids is contained in the myths of Wales and Ireland. These were written in the vernacular (that is, in Welsh or Irish, rather than in Latin) during the medieval period. The Ulster Cycle of prose, compiled some time between the seventh and twelfth centuries AD, but probably deriving from much earlier oral tradition, contains what are arguably the earliest vernacular mythic texts. One of the most prominent characters in these tales is the Druid Cathbadh, who was attached to the court of King Conchobar of Ulster. Another group of texts, known as the Mythological Cycle, compiled in the twelfth century, describes a divine race of Ireland, the Tuatha Dé Danann, who were steeped in Druidic lore and magic. In a different set of stories, the Fenian Cycle, also written down in the twelfth century, the central character is Finn, leader of the Fianna, a war-band sworn to guard the king. As a child, Finn was reared by two holy foster-mothers, one described as a ' wise woman ', the other a Druidess. Finally, the early chronicles of Saint Brigit allude to the rearing of the great sixth-century Irish saint in a Druid's household; and chronicles of Saint Patrick refer to his encounters with Druids.

References to the Druids in the early Welsh literature are more scarce. However there is an interesting parallel to the Life of Saint Brigit in the twelfth century Life of Saint Beuno, the sixth century canonised abbot, which contains an account of his death: on his arrival at the gates od Heaven, he encountered Saint Peter, the other apostles and the Druids. Only literary evidence, whether from the Classical world or from the vernacular mythic tradition, can give us direct evidence of the Druids. These documents alone mention them by name: we have no inscriptions or ay images which can certainly be identified as Druidic. Thus any attempt to use archaeological material as evidence for the Druids must be made with an awareness that we are dealing with supposition rather than fact.. Perhaps the most important archaeological document which can be quite firmly linked with the Druids is the Coligny calendar, an incomplete bronze sheet, dating to the first century AD, inscribed in Gaulish with a list of auspicious and inauspicious days. This was clearly a religious text which priests may have consulted when making their predictions Other archaeolgical material relates not specifically to the Druids but to priests or cult officials in general. There is evidence of their presence in sacred structures, sacrificial deposits and regalia such as headdress, badges of office and sceptres. Certain images in stone and bronze may represent clergy rather than the gods themselves, but in the absence of accompanying dedications, we cannot be sure.