In modern scholarship, the "late" period of the Roman army begins with the accession of the Emperor Diocletian in AD 284, and ends in 480 with the death of Julius Nepos, being roughly coterminous with the Dominate.

The Imperial Roman Army was the military land force of the Roman Empire from 27 BC to 476 AD,The Imperial Roman Army, Yann Le Bohec, Routledge, 28 Oct 2013 and the final incarnation in the long history of the Roman army.

By the size of the Roman army is meant the changes (increases and reductions) in the number of its contingents: legions, auxiliaries, Praetorian cohorts, Urban cohorts, vigiles, and naval forces over the course of twelve centuries – from 753 BC to AD 476 (the Fall of the Western Roman Empire).

The Roman army of the mid-Republic, also called the manipular Roman army or the Polybian army, refers to the armed forces deployed by the mid-Roman Republic, from the end of the Samnite Wars to the end of the Social War.

The Roman army of the late Republic refers to the armed forces deployed by the late Roman Republic, from the beginning of the first century BC until the establishment of the Imperial Roman army by Augustus in 30 BC.

The early Roman army was deployed by ancient Rome during its Regal Era and into the early Republic around 300 BC, when the so-called "Polybian" or manipular legion was introduced.