The Celts present one of antiquity's most striking examples of cultural continuity across vast geographic distances: from Ireland to Anatolia, peoples sharing Celtic language and material culture created a network that was recognizable as coherent even though it lacked political unity.

The Celts are better defined by language and material culture than by ethnic or political identity. Celtic languages — the Insular Celtic of Ireland and Britain, and the Continental Celtic of Gaul, Iberia, and central Europe — share enough features to indicate a common ancestry, probably in the Bronze Age central European cultures that archaeologists associate with the Hallstatt and La Tène material traditions.

Celtic society, known primarily from archaeology and from Greek and Roman ethnographic accounts, was organized around kinship groups, warrior aristocracies, and the religious specialist class of druids. Caesar's description of the druids in the Gallic Wars — as philosophers, educators, and religious specialists who spent twenty years in training and kept their knowledge oral — is the most detailed ancient account, though Caesar had political reasons for both admiring and distorting what he observed.

The Celtic artistic tradition, particularly the La Tène style, is characterized by curvilinear ornament, asymmetric composition, and a distinctive treatment of animal and human forms that influenced early medieval Irish and Northumbrian art. The Book of Kells and the Lindisfarne Gospels carry forward a visual tradition that can be traced back to Iron Age Celtic metalwork. The cultural continuity between pre-Christian Iron Age Celtic art and Christian medieval insular art is one of the more remarkable artistic survivals in European history.