The Centurion was normally responsible for a century of men which during the 1st and 2nd Centuries AD was made up of ten, eight man groups totalling 80 personell in all. It was the Centurions responsibility to ensure the discipline of the men under him both on and off the battle field.

It is believed that like his earlier Greek and later Byzantine counterparts the Centurion would be expected to fight in the front with his subordinates, usually to the front and right of the formation, although no true evidence is made in support of this it is generally accepted to be so because the Greeks fought this way to try and halt the tendency of the phalanx to lead to the right. It is a mark of the Roman officer that generally they were expected and found to be in harms ways during battles as can be attested by the grave writing tendency to include hand to hand fighting and other virtus events ahead of other attributes the dead may have possessed.

 Becomming a centurion was not a straight forward case of passing exams and gaining the title as it is in many modern armed forces. A centurio was either directly commisioned from civilian life, usually from the ranks of the urban elite of the curiales or decuriones (NB in civilian context, thus city council members, no cavalry officers) or the equestrian order. Others received their commission after service in the praetorian guard or after serving in various posts as NCO's, usually having held all of the duplicarii posts (ie optio, signifer and cornicularius apparently in random order) or other specialist positions (eg as beneficiarius). In addition some of the legionary centuriones were promoted from posts as decurio in the auxiliary cavalry or the imperial horse guard. There are no discentes attested for the centurionate, soldiers awaiting their promotion being designated ad spem ordinis or candidatus. In exceptional cases centurions were elected by the troops, a practice that was heavily frowned upon by senatorial historians like Tacitus. Civilian appointees may have been enrolled previously in one of the paramilitary iuventus organisations, whose main activities seem to have been in teaching upperclass youths horse riding skills and weapons handling rather than military theory, though membership of such an organisation is not known to have been a prerequisite.