Praetorian Guards were the elite bodyguard of Roman emperors.
Augustus in 27 B.C. organized the corps into 9 cohorts, but Caligula (reigned 37 to 41 A.D.) had 12, Vitellius in 69 A.D. had 16, and Domitian (reigned 81 to 96 A.D.) reduced them to 10, at which total they remained until Constantine I the Great deactivated them in 312 A.D.
Each cohort presumably numbered 500 soldiers (excluding auxiliaries), until Severus (reigned 193-211 A.D.) doubled the strength. For two centuries the guards, whose camp was along Rome's northern wall, were recruited from Italy and the Romanized provinces, but thereafter non-Romans were enlisted because of their superior loyalty.
Praetorians were considered select troops, since they served only 16 years, drew thrice the legionary pay, wore distinctive armor, and received frequent donatives.
Their commander (prefect) became a principal minister of state and his praetorians occasionally made and unmade emperors.