A formative period for Spanish history began with the Roman conquest at the end of the third century BCE. Roman political control thereafter expanded over most of the peninsula and lasted through the fourth century CE, as Iberia became one of the most Romanized portions of the empire. Fundamental components of Iberian life were set in place and continued to influence later developments long after the Roman Empire ended. Roman innovations and foundations underlay many medieval and modern developments in Spain and remain strong and apparent in Spain today in such areas as language, law, and religion.

Initially, the process of Romanization in Iberia proceeded slowly, as the Romans followed the pattern that their ancestors had set in taking over the Italian Peninsula, making treaties with groups who agreed to join them voluntarily and conquering those who chose to resist. Nonetheless, it took about 200 years for the Romans to establish full control in Iberia, whereas they conquered Gaul in a decade. The difficult terrain and the traditions of local rule in Iberia made the Roman conquest extraordinarily difficult, and there was scarcely a year in two centuries that did not see fighting in one or more regions.

Soon after the conquest, the Romans divided the area into two parts: Hispania Citerior and Hispania Ulterior (Closer and Farther Spain, respectively). All they really controlled by the end of the Punic Wars was a band along the Mediterranean coast and beyond Gibraltar as far as Cádiz. That was the part of the peninsula with the greatest number of towns and cities, and the part longest influenced by the other Mediterranean civilizations. Because larger political entities did not exist previously in Iberia, the Romans had to deal separately with one small group after another. They focused on cities and towns in this segmented expansion, making treaties with existing towns and tribes that agreed to join the Roman world, and giving their inhabitants affiliate Roman status. Towns or regions that resisted they forced into submission, often through long wars. This was the case with Numantia, which fell only after a lengthy siege.

Though the Romans had become involved in what they called Hispania only to thwart the ambitions of Carthage, they were quick to recognize the advantages the peninsula offered.