The praefectus vigilum (lit. 'prefect of the watchmen', pl.: praefecti vigilum) was, starting with the reign of the Emperor Augustus, the commander of the city guards in Rome (cohortes vigilum or vigiles), whom were responsible for maintaining peace and order at night--a kind of fire and security police. Although less important than the other prefects, the office was considered a first step in order to reach an important position in the imperial administration.

The offices of the praefectus vigilum were located in the Campus Martius, perhaps in the quadriportico of the theatre of Balbus (along the via Lata), inside the barracks of the First Cohort of Vigiles (Latin: statio primae cohortis vigilum). The reason to think that is that all the dedications found in the remains of these barracks are inscribed in the name of the prefect. It was in this building that the praefectus vigilum had his offices and his courtroom and it was from there that he left every night to carry out the statutory rounds.

Without the participation of the Senate or the Roman people, the praefectus vigilum was appointed by the Emperor for an indefinite period and just as arbitrarily removed again. He supervised the fire prevention and fire fighting service in the city of Rome. He was also responsible for maintaining order in the streets at night. The Prefect was vested with both military (he was the head of the corps) and civil (judicial) powers, the latter giving him jurisdiction over arsonists, burglars, thieves, robbers and fences. He could impose light sentences (lashes or sticks) but important cases were transferred to the responsibility of the prefect of the city (praefectus urbi). His staff therefore consisted of two types of personnel, the first military (officers and non-commissioned officers) and the second civilian (regular administrative staff). Each night, the prefect must personally lead one of the surveillance rounds and circulate in the town throughout the night. He had the right to search private homes to check whether fire safety standards were being respected. The praefectus vigilum was assisted by a subpraefectus and tribuni, who on their part were backed by civil servants. From the end of the fourth century onwards, the organisation of the Corps was modified. The prefect of the watchmen was placed under the orders of the prefect of the city, of whom he became one of the heads of department.

The Prefect of Vigils was not a magistrate, but a member of the equestrian order and an imperial commissioner (prefect) appointed by the emperor. Later this position was also filled by senators. His position was considered inferior to the other prefectures of Rome, like the praefectus urbi, annonae and praetorio, which were considered more prestigious. It was considered a first step for knights seeking to climb the ladder of public offices.

From the 2nd century onwards, under the reign of Trajan, the prefect had more and more legal powers and the presence of a sub-prefect became necessary. One of the first sub-prefects was Gaius Maesius Tertius, appointed in 113. The sub-prefect of the vigiles had his own offices, similar to those of his chief of service, but with a smaller staff. When necessary, he replaced the prefect. The position of sub-prefect represented one of the centenarii of the career of the procurator (procurator centenarius was the governor of a part of a province). The position was accessible through the regular career but primipili could also access it directly. According to Hans-Georg Pflaum, a subpraefectus vigilum got a wage of 100,000 sestertii.

Under Claudius, a corps of vigiles was detached from the garrison of Rome and stationed in the port of Ostia. This detachment was temporarily placed under the orders of a sub-prefect during the 3rd century, who was himself under the orders of the prefect of the Vigiles of Rome. There would then have been two sub-prefects of the vigiles, one in Rome and the other in Ostia.

During the Republic, the firefighting service was limited to groups of slaves made available by wealthy individuals, such as one set up by Marcus Licinius Crassus and one set up by Marcus Egnatius Rufus while aedile in 21 BC. Their number, less than 600 "firemen", remained very insufficient in the face of the risk of fire which increased with the accelerated urbanisation of the city from the end of the Republic.

The night watches were reorganised by Augustus in 6 AD because of the frequent fires in the city, which were especially dangerous at night. He created a larger and better (militarily) organised corps, the vigiles urbani. They were led by the praefectus vigilum and divided into seven cohorts (of 560 or 1,000 men each), each covering two of the fourteen city districts (regiones), and each led by a tribunus. The vigiles were mainly recruited among the freedmen, and therefore enjoyed a lower standing than the regular army, but the corps was also open to citizens of the provinces of the Empire and later to Roman citizens.