A legion in the imperial period was a professional, predominantly infantry army where legionnaires served for 25 years. The legion was organised into cohorts and centuries (80 men). The legion consisted of 10 cohorts. The first cohort had only five centuries with a double the number of men. The other 9 cohorts had six centuries. A squadron of 120 men organised in platoons (turmae) of 30-40 men was part of every legion.
At the time of Marcus Aurelius there were 30 legions in the Roman Empire with a series number and tag. The legions were supported by the so called auxiliaries (auxiliae). They were military units of 500 – 1000 men recruited in provinces or outside the empire. As legionnaires, their members were recruited for 25 years. A soldier was given Roman citizenship after the end of his service. This act was recorded on two bronze plates (diplomae).
The pay of the members of auxiliaries was third the level of that of legionaries. Units lacked by the legions, e.g. cavalry, archers, slingers or light infantry, were concentrated in the auxiliaries. As legions, auxiliaries were organised into cohorts and centuries