The Belgae were a large confederation of tribes living in northern Gaul, between the English Channel, the west bank of the Rhine, and the northern bank of the river Seine, from at least the third century BC. They were discussed in depth by Julius Caesar in his account of his wars in Gaul. Some peoples in southern Britain were also called Belgae and had apparently moved from the continent. T. F. O'Rahilly believed that some had moved further west and he equated them with the Fir Bolg in Ireland. The Roman province of Gallia Belgica was named after the continental Belgae. The term continued to be used in the region until the present day and is reflected in the name of the modern country of Belgium.

Gaul was a region of Western Europe first clearly described by the Romans, encompassing present-day France, Belgium, Luxembourg, and parts of Switzerland, the Netherlands, Germany, and Northern Italy. It covered an area of 494,000 km2 (191,000 sq mi). According to Julius Caesar, who took control of the region on behalf of the Roman Republic, Gaul was divided into three parts: Gallia Celtica, Belgica, and Aquitania.