The term 'auxilia' was applied to all units other than the legions and the old allies. It included cavalry and all types of infantry. In the early empire the infantry slowly changed from bands fighting in their own fashion, often under their own leaders, to units of similar size and organisation, commanded by equestrians (equites). These units, which were called cohorts, came to be modelled on the legionary cohort, i.e. six centuries- comanded by centurions- and consisting of about 500 men.

Their equipment became standardised, and on Trajans Column the auxiliary infantry fight in much the same manner as the legionary. Archers were also formed into cohorts; slingers continued to be used, though not apparently in seperate formations. In the second half of the first century AD bigger versions of these units were introduced, at about the same time that the first cohort in a legion was increased in size.

They were composed of ten centuries, corresponding to the five double centuries of the remodelled first legionary cohort. The auxiliary cohorts were originally raised in the provinces. Some were posted away, but others were stationed close to home, most notably on the Rhine. A dangerous mutiny on the Rhine during the civil war in 69 led to a transfer of units away from that area, but the new ones coming in were kept up to strength by local recruiting, which was customary for the auxilia everywhere in the empire. Only for the specialist oriental archers were recruits sought in the lands where the units~ were originally raised. Unit names therefore only refer to the original area of recruitment.

Below the rank of centurion the auxiliary cohort had a similar but simpler structure to that of the legion. The centurions were drawn from the ranks and spent their lives in the same unit. This was commanded by an equesrian prefect, as the first step in a career which might lead him, via the legionary tribunate or a command of a 1000- strong auxiliary cohort, to a cavalry command. He would come to army life fresh from a civilian, possibly alter holding the senior magistracy in his home town in his twenties or thirties. The post was in the province that he had governed.

The larger units were commanded by tribunes who had previously commanded one of the ordinary cohorts. They were selected for these posts by the emperor rather than by the governor and seem to have been the most promising of the infantry prefects.

The auxiliary infantryman served in the army for 25 years. On discharge he recieved a grant of Roman citizenship which was inscribed on a pair of bronze sheets known as a diploma. Hundreds of examples have been found, and they are the main source of our knowledge of the auxiliary units. The auxiliary infantryman was probably paid only about a third of the amount that of a legionary. Like the legionary, the auxiliarv soldiers were not allowed to marry. How ever, they did form lasting associations which were regarded as a marriage by all concerned, and the emperor accepted this by granting citizenship to their children as well as to the auxiliaries themselves.