Their territory spanned Devon and Cornwall with further holdings in Somerset and Dorset. They were possibly the most powerful of all the British Celtic tribes. It is also possible that the Dumnonii held some Breton land at some point. The name of Dumnonia in Celtic is Dunein.
Originally their capital would have been at Exeter (known to the Romans as "Isca" but to the Britons as "Caeresk"), and there is evidence that the Celts continued to live in Exeter until (at least) the tenth century as equals with the newer Saxon arrivals. A part of Exeter retained the title 'Little Britain' until the eighteenth century.
Dumnonia was believed to be effectively a collection of sub-kingdoms.