The origin of Celtic culture is unclear. Some scholars have suggested the existence of a proto-Celtic Indo-European people in central Europe as early as the third millennium B.C.E. Others date the origins to the second millennium B.C.E. More solid claims date the beginnings to the central European Hallstatt culture of the ninth century B.C.E. The aristocratic-warrior La Tène culture of Europe in the fifth century B.C.E. is generally considered to have been Celtic. We know from the Greek writers Herodotus and Hecataeus of Miletus that Celtic peoples, speaking related languages, lived in most areas of central and western Europe by 500 B.C.E. Early in the fourth century B.C.E. Celtic tribes, called Celtae or Galli by the Romans, sacked Rome. Not long after that Celts, now called Galatae, moved as far east as Asia Minor, where they founded the city of Galatia. Meanwhile, to the west, Celtic migrations to Brittany in France and the Islands of Great Britain and Ireland took place between the fifth and first centuries B.C.E. Julius Caesar led Roman wars against the Celts in Britain beginning in about 50 B.C.E. and against Celts in the areas of Germany and France known as Gaul somewhat earlier. Caesar never reached Ireland and achieved only minor success in Wales and Scotland. To this day Celtic people of the Gaelic linguistic branch dominate the populations of Ireland and Scotland, while Brythonic Celts live in Wales, Cornwall, and Brittany. The word British is derived from the Breton Bryttas, a name for Celts living in the British Isles.

More specific knowledge of the Celts, at least from their own perspective, is limited because the Celts lacked a practical writing system; their stories and traditions were passed down orally. The early written history of the Celts, therefore, comes from writers such as Herodotus and, of course, Julius Caesar, who wrote about the Continental Celts—the Gauls—in his famous history of the Gallic wars. When we speak of Celtic history and mythology today, we speak of the “Continental Celts,” about whom we have limited knowledge, mostly from their enemies, and the “Insular Celts” of Ireland and Great Britain whose traditions, especially in Ireland and Wales, were well preserved by later writers with a genuine respect for Celtic culture.