The Nervii were one of the most powerful Belgic tribes of northern Gaul at the time of its conquest by Rome. Their territory corresponds to the central part of modern Belgium, including Brussels, and stretched southwards into French Hainault. During their first century BC Roman military campaign, Julius Caesar's contacts among the Remi stated that the Nervii were the most warlike of the Belgae. In times of war, they were known to trek long distances to take part in battles. Being one of the northerly Belgic tribes, with the Menapii to the west, and the Eburones to their east, they were considered by Caesar to be relatively uncorrupted by civilization. According to Tacitus they claimed Germanic descent. According to Strabo they were of Germanic origin.
They are mentioned as Nervii by Caesar (mid-1st c. BC) and Orosius (early fifth c. AD), Neroúioi (Νερούιοι) by Strabo (early first c. AD), Nerui by Pliny (1st c. AD) and the Notitia Dignitatum (5th c. AD), Nervios by Tacitus (early second c. AD), and as Neroúsioi (Νερούσιοι) by Ptolemy (2nd c. AD).
The ethnonym Nervii probably stems from the Western Indo-European root *ner-, meaning 'man' (cf. Middle Welsh ner 'lord, chief'). It appears to be cognate with the Latin cognomen Nerva.
According to Xavier Deru, the core region of the Nervii was equivalent to the medieval pagus of Hainaut, the region of the Haine river, the upper Sambre, and greater and lesser Helpe rivers. To the north of the Haine, it also included what would become the medieval pagus of Brabant, making its northwestern border on the Scheldt (French Escaut, Dutch Schelde) river. A large population occupied the southern territories, near the river Sambre with the biggest being at Avesnelles, near Avesnes-sur-Helpe.
An oppidum found near Asse may have belonged to them but it was isolated and near to the territory of the Menapii. In the south the Nervians stretched no further than the forests of Arrouaise and Thiérache South of them were the Viromandui, south of Cambrai, and the Remi.