The army was an institution of central importance throughout Roman imperial history. This chapter talks about two sources Notitia Dignitatum, which allows one to see something of the formal organization of the empire's military forces; and History of Ammianus Marcellinus, which enables to observe the army in action, in its military capacity and in its wider political and social context. The main theme in the organizational evolution of the field army after Constantine's death is regionalization. The army was a consumer of human resources, and emperors of the period do seem to have had difficulties finding sufficient recruits. From the advent of monarchy under Augustus, maintenance of a good relationship with the army was always one of the most important political priorities of emperors. The starting-point in assessing the effectiveness of the limitanei must be the question of whether they were in origin a peasant militia, given land to farm while they performed military service.