The Roman senate appointed the Legatus or commander of the legion. They also appointed the Tribuni who served on the Legatus' staff. This could result in commanders with little field experience. Since there were no military academies the only way to gain real command experience was on the battlefield. In order to maintain the high levels of troop proficiency and discipline required of a standing professional army the Romans depended on the Centurions. The Centurions were professional soldiers, and they provided the core of experienced commanders needed to run the legions. Because they were Centurions were essential, and they had a high mortality rate, the common legionnaire had a valuable path for career advancement.

The Centurions were commissioned field officers who were generally recruited from the ranks. To become a Centurion one had to serve a minimum of 15 years. He also had to have served as a junior officer (NCO) on a Centurion's staff. But, most importantly, he had to have shown intelligence in the ability to lead and discipline men. The Centurion's primarily duty was the command of his century or cohort and the administration of discipline to the rank and file under his command. He also issued orders for guard duty and work details. He was better paid than the rank and file, receiving 3,700 denarius annually during the reign of Augustus. This was 5 times the rate of pay of the legionary serving in the Praetorian Guard.

Signs of his rank were the leg greaves and a crested helmet. These elements made him easy to spot on and off the battlefield. The common legionnaire was not issued greaves and only wore his helmet crest during ceremonial occasions. Another symbol of his office was the vine staff mainly used in the discipline of troops. The Centurion was also assisted in his duties by a staff of junior officers and an Optio, Signifer, and Tesseraus. The Optio was an officer in training as the centurions understudy. If the Centurion fell in combat the Optio took his place. The Signifer served
as a standard bearer, and the Tesseraus gave and received the daily watchword.

The order of promotion for the rank and file Centurion started at the 6th century of the 10th cohort and proceeded from century to century. Holding the posts of Hastatus Posterior, Hastatus Prior, then, Priceps Posterior, Priceps Prior, and finally Pilus Posterior. After which he would go to the next cohort. This order was repeated all the way up to the 2nd. Cohort. These Centurions were classified as Centuriones Ordinarus. Once he was promoted to the position of Pilus Prior, which was the Centurion in command of any cohort, or became a Centurion of the first Cohort, he became a Primi Ordines and was eligible to be promoted to the coveted position of Primus Pilus, the first spear. The Primus Pilus was the first Centurion of the first century of the first cohort.

The Primus Pilus was the senior Centurion. It was a position of honor and he was responsible for the protection of the legionary eagle. He also served as an advisor of military matters on the staff of the Legatus and sometimes assumed the command of the legion itself. After holding this rank for a year the Primus Pilus could be promoted to the rank of Tribuni Militaris commanding a cohort of Vigils in Rome. The Vigils included the urban cohorts and the Praetorian Guard. The Primus Pilus could also be promoted to the rank of Prefectus Castrorium, the camp prefect. The camp prefect was not directly attached to the legion but the rank placed him third in the chain of command. His duties included camp defense, the maintenance of the camp stores and weapons, and the supply of the forward units. After serving his term as camp prefect he became eligible for the civil service post of the Procuratorship, a financial officer who served under the provincial governor, or sometimes as the provincial governor of an imperial province.

The Primus Pilus was also eligible to enter what was called the Equestrian Cursus. In order to do so he had to hold two additional military posts commanding cohorts in the Auxuilia, after which he received his equestrian status. He quite possibly returned to the legions as Tribunis Angusticavii. The Equestrian Cursus was a "fast track" to promotion for a person of equestrian rank. This person could secure an appointment directly into the Primi Ordines. This ensured he would be promoted quickly and spend much less time among the rank and file, placing him a better position to reach the rank of Primus Pilus or beyond.

Ideally you could start at the bottom and work your way through the ranks. The possibility of advancement and the high rate of pay encouraged many Centurions to become career soldiers and serve in the legions long after their tour of duty had expired. The promotion scale was encumbering, but if you were a person of intelligence and ability, and if you could survive the 20-year term of service, there was an opportunity to achieve a high-ranking position in the legion and change your social status.