Occupation at Bainbridge began in the governorship of Agricola. Little is known of the first fort; the visible remains represent a successor fort, established in c.a.d. 85 at the earliest, abandoned under Hadrian and Antoninus Pius, and reoccupied in c. a.d. 160. In the early Severan period, the size of the unit at the fort seems to have been greatly reduced in numbers, and a suite of rooms for an officer was inserted in the principia. Extensive work by cohors VI Nerviorum which took place in c. a.d. 205–7 included the building of new principia, the relocation of the east gate, and probably the addition of an annexe, its wall described in an inscription from the site as a bracchium. The fort was held until the end of the Roman period, by which time the principia had been partly demolished to provide space for a timber building probably accommodating the commanding officer. The aedes and part of the rear range seem to have stood until the ninth or tenth century, when the former was possibly converted into a church. Knowledge of this sequence of occupation depends largely on the results of Brian Hartley's excavations which are published here. The main focus of the report is on the remarkable series of principia, but a review of what is known of the overall archaeology of the fort is also included in the main text. The Supplementary Material (http://journals.cambridge.org/bri) contains a more detailed analysis of some of the other excavations together with various specialist reports.