- The reforms of Diocletian take effect, dividing Britain into four provinces and separating military and civilian government.
- Establishment of initial Christian hierarchy in Britain.
- Three British bishops attend the Council of Arles.
- Julian makes Britain main granary for western Roman army.
- Picts and Irish attack northern frontier.
- Theodosius defeats invaders, builds new watchtowers from Filey to Huntcliff, re-fortifies northern frontier.
- Maximus usurps control of the Empire, taking troops from Britain and abandoning the forts at Chester and the Pennines.