The earliest written references to the Celts were made by the Greek writers Hecataeus and Herodotus, during the sixth and fifth centuries BC (600-400BC). The Greeks named them Keltoi or Galatae, and to the Romans they were known as the Celtae and Galli (or Gauls). To those living in the Classical Mediterranean world they were considered as alien and barbarian people who lived north of the Alps.

We do not know what the Celts called themselves. This pre-Christian Celtic world might best be viewed as a loose mosaic of chiefdoms and societies, and not as an empire or nation. Within them a range of related Celtic languages and dialects were spoken.

Soon after 400BC, classical writers record a large-scale migration of Celts from central Europe across the Alps into northern Italy and into eastern Europe. Rome was attacked during the 380s BC. The geographer Strabo recorded a friendly meeting between Celts and Alexander the Great in the Balkans in 335BC, whilst in 279BC the Celts are known to have looted the sacred Greek site at Delphi.