The Roman "knights" or equites where formed during the Roman republic (509 BC - 27 BC). Eques (plural equites) means "cavalryman" and the equites Romani were those who served as Roman cavalry. In these times men had to pay for their own equipment and a man with sufficient wealth to pay for the upkeep of a horse was therefore eligible into the equestrian order. However maintaining a horse and the service in the cavalry was so expensive that the state gave financial assistance. Each eques was given a horse by the state and a sum of money for their upkeep hence the name equites equo publico, "horsemen with mounts provided at public expense". The equestrian order was one step below the senatorial order. No obligation lay upon knights to serve the state, but if they chose an official career, a wide range of the slightly less important executive and military positions lay open to them. Those men were enrolled in 18 special centuries of the centuriate assembly. Anyone who could afford to serve in the cavalry but didn't do so was informally called a eques Romanus, which was thus simply an indication of wealth. By the third century BC, it was reasonably clear that the Roman cavalry was not very effective militarily, and the Romans began to recruit cavalry units from their allies (Gauls, Spaniards, Numidians). Since the equites Romani no longer needed to serve as actual cavalry, these wealthy individuals could be used as officers of the legion, tribuni militum, and they became a much more political organization.

      As part of the formalization of social distinctions that went on under Augustus the equestrian order became once again a more military based organization. The term equites was officially limited to the equites equo publico, although all who possessed enough wealth were still considered to belong to the equestrian order. The criteria for becoming such an eques Romanus was defined on a monetary basis and required the possession of property worth at least 400,000 sesterces. The equites distinguished themselves from the plebs by several insignia (like a plain gold ring and a toga with a small purple stripe). As an additional honor, they had special seating at public events. Those in the equites equo publico, where reformed on a strictly military basis, they were divided into six turmae (cavalry divisions) that replaced the 18 centuries of the centuriate assembly of the Republic. Young men of senatorial rank would serve as the "commanders" of these turmae. In order to provide a supply of competent officers, each eques was required to fill certain subordinate posts, called militiae equestres.
These were:
                     (I)    Praefect of an auxiliary cohort (consisting of 500 - 1000 non-Romans)
                     (II)   Military tribune of a legion
                     (III)  Praefect of an auxiliary cavalry squadron (500 - 1000 men)

   ,this order being as a rule strictly adhered to. To these three Septimius Severus added the centurionship. After the completion of their preliminary military service, the equites were eligible for a number of civil posts, chiefly those with which the emperor himself was closely concerned. Jobs like procurator, the praefectures of the corn supply, the fleet, the praetorian guard and the governorships of recently acquired provinces. The provinces were mostly small with as major exception Egypt.

      Theoretically, the equites equo publico were young men. Under the Republic they were supposed to hand back their public horse if they could no longer ride it or take care of it. And as a youthful organization, the equites equo publico could bestow the title PRINCEPS IUVENTUTIS (Leader among youth or First among the young men). From the time of Augustus this title was bestowed on various sons of the ruling emperors as a mark of distinction and a sign of being heir to the throne. Augustus also reintroduced the old Republican custom of reviewing the equites equo publico on the 15th of July. This full-dress procession (transvectio) from the temple of Mars to the Capitol was started in 304 BC by the censor Q. Fabius Maximus Rullianus to commemorate the miraculous intervention of Castor and Pollux at the battle of Lake Regillus.

      Under the empire the power of the equites was at its highest in the time of Diocletian 284 - 305 AD. After that their influence slowly diminished and as a consequence of the transference of the capital from Rome to Constantinople, they sank to the position of a mere city guard, under the control of the praefect of the watch. Their history may thus be said to end with the reign of Constantine the Great in 324 - 337 AD.