household troops of the Roman emperors. The cohors praetoria existed by the 2nd century BC, acting as bodyguards for Roman generals. In 27 BC the emperor Augustus created a permanent corps of nine cohorts, stationing them around Rome; in 2 BC he appointed two equestrian prefects to command them, but in AD 23 Tiberius' powerful prefect Sejanus (Sejanus, Lucius Aelius) became their sole commander. He concentrated them in fortified barracks outside the walls of Rome, gaining significant political influence for them.

Subsequently, they generally participated in appointing emperors and were responsible for the accession of Claudius (41); the disorders of 68–69; the lynching of Domitian's murderers (97); and the murders of Pertinax (193), Elagabalus (222), and Balbinus and Maximus (238). Septimius Severus reorganized the guard in 193, recruiting its members from the legions. Constantine I disbanded them in 312.