One of the major sources of evidence for the military history of the Roman Empire, particularly for those provinces for which the literary record is defective, is the shifting pattern of forts and fortresses, established and abandoned in response to changing military situations, to advance and retreat and to consolidation and the establishment of frontier control. In order to have any hope of understanding the role of individual forts or groups of sites it is necessary to know not only the detail of their internal layout but also the nature of the troops who occupied them. Several studies have been devoted to the problem of the elucidation of fort garrisons, notably those of Richmond in 1955, Breeze and Dobson in 1974 and, most recently, that of Hassall in 1983. However, these studies have all concentrated on sites of the Flavian period and later and have been concerned almost exclusively with the accommodation of auxiliaries, attempting to identify the forts occupied by the various types of unit which had emerged by that period, alae, quingenary and military, and cohorts, quingenary and milliary, peditate and equitate, each with its individual requirements in terms of numbers, size and arrangement of barracks, stables and stores.