The Celts originally inhabited an area in southern Germany and Bohemia. By the end of the 5th century BC they had expanded into the Iberian peninsula; in 390 BC they sacked Rome. In the east they went as far as Anatolia. In the west they migrated to Britain in the 5th century BC and Ireland in the 3rd century BC.

A great deal may be learned about the Celts from the archaeological materials left behind in the various countries where their culture dominated for several centuries. Most of the written documents of Celtic culture and religion are from Ireland and date from the 12th century AD, when they were written under Christian aegis.

As in the other Indo-European cultures, a clear tripartite structure appears in Celtic societal organization. The principal divisions are the king, the warriors, and the cattle herders. The religious hierarchy is also tripartite, consisting of the priestly Druids, who also served as administrators; the vatis or filidh, experts in magic and divination; and the bards, who are concerned with oral literature and prose poetry.

As a culture the Celts display counteracting tendencies: they seem to be autonomous, anarchic, and concerned for local traditions, but a basic unitary character is manifested in their social organization and mythological histories.

The Celtic pantheon is difficult to discern. The names of several hundred gods are known, but the majority appear to be local deities. During the Roman period, many Celtic deities were identified with Roman gods. One of the most important, called Lug in Ireland, was identified with Mercury.