The equestrian order (ordo equester) was larger than the senatorial order, numbering twenty thousand even in Augustus' reign. This number increased through the first two centuries of the imperial age, owing to the growing influx of provincial subjects into the order in question.

Membership of the equestrian order was not necessarily hereditary: there were cases of the son of an equestrian not holding the same rank. In certain equestrian families, consequently, they were unable, unlike the senatorials, to sustain their membership across several generations. Their male descendants might, however, rise to the senatorial order.

The social composition of the equestrian order was heterogeneous. It included members of humble origin, as well as some who had come from the freedmen. Economic condition, links forged with powerful Romans, and military service in person were the three basic factors playing a role in beoming a member. During the early Imperial Age, the rank of equestrian was also granted to leading executives of local provincial noble clans, as can be seen from inscriptions from the provinces of Hellas and Asia.

Unlike the senatorial order, the equestrian order was characterized by greater racial heterogeneity, since it was easier for dwellers in the provinces to scrape together the minimum sum of money needed for entry into it. In the 1st century A.D., both the provincial equestrians and the senatorials were for the most part from urnaized provinces like Asia in the east. The founding of Roman cities in various provinces led little by little to the creation of economically well-off families actuated by the Roman mentality, their members able to be rewarded with the 'public horse' (equus publicus).