There is much less known about the structure of the army in the late Empire than the 'classic' legion structure of the early Empire. From the late first century onwards, the boundaries of the Empire started to become fixed frontiers. The increasing sophistication of the tribes outside the Empire, caused in some ways by the presence and success of the Empire itself, meant that wars in the later Empire, when they erupted, were hard-fought affairs against increasingly larger confederacies of tribesmen, armed with weapons as effective as those of the Roman army. Often, the army found itself too thinly spread along a frontier to meet the threat of a powerful enemy. To counter this problem, standing field armies, at nodal points away from the frontier, were created to act as a mobile reserve. The legions gradually fragmented as cohorts were detached and sent to fill breaches in the defences. The structure of the army evolved, giving greater emphasis to the cavalry for increased mobility; the final system emerged under Constantine:

• The limitanei - Frontier troops, made up of old-style alae and cohorts, supported by the legions, plus new style cavalry and infantry auxilia.
• The comitatus - Higher grade field armies, consisting of regiments, some derived from the existing legions.

Originally the comitatus was under the Emperor's command, but during the fourth century field armies were created under the command of Dux and Comes within provinces. The British section of the Notitia Dignitatum (a list of Roman civil and military posts, originating from c.400) records the Comes Britanniarum as commanding six cavalry and three infantry units of the comitatus. Units had become smaller, with field army regiments at around 1,000 for infantry and 500 for cavalry and the limitanei at around 300. Specialist artillery units equipped with ballistas and bolt throwers appeared. Units continue to have names that reflect their origins:
Equites Dalmatae Branudunenses were raised in what is now Croatia around the middle of the third century and based at the Saxon Shore fort of Branodunum (Brancaster, on the Norfolk coast). However sources increasingly use shorter names and many late regiments have titles combining 'unit' - Numerus - and their location or origin, for example:
Numerus Turnacensium, originally based at Tournai and listed in the Notitia as the garrison of Lympne. The old-style regional fleets disappear under the late Empire. It is possible the Classis Britannica may have been split into smaller units attached to the coastal forts and operating, as a fighting/reconnaissance force, in concert with the fort's garrison. Presumably the Classis Anderetiana, recorded in the Notitia Dignitatum as being based at Paris but originating at Anderida (Pevensey), was such a unit.