The story of the Celts begins in prehistory, the time before written records were kept. Originating in what is now Eastern Europe, the Celts appear to have moved west along the main trading routes of that time, especially the river Danube, into modern Germany, Austria, Switzerland and France.
By the beginning of the classical period (about 500 BCE), they were a large group of tribes and races spread over a wide area of Europe, from Scotland and Ireland in the north-west to Russia in the east, and to the Mediterreanean in the south. The earliest archaeological evidence associated with the Celts places them in what is now France and western Germany in the late Bronze Age, around 1200BCE. In the early Iron Age, they are associated with the Hallstatt culture (8th to 6th century BCE), named for an archaeological site in what is now Oberösterreich (Upper Austria).
By the time the existence of the Celts was recorded by the Greek writer Ephorus in the fourth century BCE, they were so numerous that he named them as one of the four great barbarian peoples in the world. The other three were the Libians in Africa, the Persians in the East and the Scythes who lived in Europe as well.
In the 4th century BCE, the Celts invaded the Greco-Roman world, conquering northern Italy, Macedonia, and Thessaly. They sacked Delphi in 279, plundered Rome in 390, and penetrated Asia Minor, where they were known as Galatians. The "Cisalpine Gauls" of northern Italy were conquered by the Romans in the 2nd century BCE; Transalpine Gaul (modern France and the Rhineland) was subdued by Julius Caesar in the 1st century BCE, and most of Britain came under Roman rule in the 1st century CE. In the same period, the Celts of central Europe were dominated by the Germanic peoples. They probably began to settle in the British Isles during this period. Between the 5th and 1st centuries BCE, their influence extended from what is now Spain to the shores of the Black Sea. This later Iron Age phase is called La Tčne, after a site in Switzerland.
The unity of the Celts was not that of a nation or empire in the Greek or Roman sense, but was more cultural in nature, with no clear central authority. Celtic tribes dominated a huge area, and had their own individual identities, but they shared many common roots including similarities in language, religion, and lifestyle. They probably called themselves something similar to Celts, from which the Greeks got their word for 'stranger' - keltoi: the name given to these people by Herodotus and other Greek writers. To the Romans, the Continental Celts were known as Galli, or Gauls; those in the British Isles were called Britanni.
In medieval and modern times the Celtic tradition and languages survived in Brittany (western France), Wales, the Scottish Highlands, and Ireland.