The Roman Republic conquered and occupied territories in the Iberian Peninsula that were previously under the control of native Celtic, Iberian, Celtiberian and Aquitanian tribes and the Carthaginian Empire. The Carthaginian territories in the south and east of the peninsula were conquered in 206 BC during the Second Punic War. Control was gradually extended over most of the Iberian Peninsula without annexations. It was completed after the end of the Roman Republic, by Augustus, the first Roman emperor, who annexed the whole of the peninsula to the Roman Empire in 19 BC.

The Celts or Celtic peoples were a collection of Indo-European peoples in Europe and Anatolia, identified by their use of Celtic languages and other cultural similarities. Major Celtic groups included the Gauls; the Celtiberians and Gallaeci of Iberia; the Britons, Picts, and Gaels of Britain and Ireland; the Boii; and the Galatians. The relation between ethnicity, language and culture in the Celtic world is unclear and debated; for example over the ways in which the Iron Age people of Britain and Ireland should be called Celts. In current scholarship, 'Celt' primarily refers to 'speakers of Celtic languages' rather than to a single ethnic group.