A legatus (often anglicized as legate) was equivalent to a modern general officer in the Roman army. Being of senatorial rank, his immediate superior was the dux, and he outranked all military tribunes. In order to command an army independently of the dux or provincial governor, legates were required to be of praetorian rank or higher; a legate could be invested with propraetorian imperium (legatus propraetore) in his own right. Legates received large shares of the army's booty at the end of a campaign, and for this reason the position was a lucrative one, and could often attract even distinguished consulars (e.g., the consular Lucius Julius Caesar volunteered late in the Gallic War as a legate under his first cousin, once removed, Julius Caesar).

The men who filled the office of Legate were drawn from among the senatorial class of Rome. There were two main positions; the legatus legionis was an ex-praetor and the man in command of one of Romes elite legions, while the legatus propraetor was an ex-consul, who was given the governorship of a Roman province with the magisterial powers of a praetor, which in some cases gave him command of four or more legions.

This rank was also the overall Legionary commander. This post was generally appointed by the emperor. The person chosen for this rank was a former Tribune and held command for 3 or 4 years, although he could serve for a much longer period. In a province with only one legion, the Legatus was also the provincial governor, but in provinces with multiple legions, each legion had a Legatus and the provincial governor (who was separate from the legions) had overall command of them all.