A Roman legion was an infantry unit consisting of heavily armed soldiers, equiped with shields, armor, helmets, spears and swords. In the early republic, the strength of a legion was about 3,000 men; there were 4,800 legionaries in the days of Julius Caesar; the twenty-five legions that defended the empire during the reign of Augustus counted more than 5,000 soldiers. They were the backbone of the Roman army, supported by auxiliary troops. Although in the third century, large cavalry units gradually superseded the legions as Rome's most important force, many of them are attested in the fourth and early fifth centuries.
The following table catalogue contains only the legions that served under the early empire. Units in italics were part of the army of the emperor Augustus.
In the fourth century, we hear of other legions, but at that moment, the legions were no longer the backbone of the Roman army. Their names are included in the following table: