The Britons, also known as Celtic Britons or Ancient Britons, were an indigenous Celtic people who inhabited Great Britain from at least the British Iron Age until the High Middle Ages, at which point they diverged into the Welsh, Cornish, and Bretons. They spoke Common Brittonic, the ancestor of the modern Brittonic languages.

The Roman conquest of Britain was the Roman Empire's conquest of most of the island of Britain, which was inhabited by the Celtic Britons. It began in earnest in AD 43 under Emperor Claudius, and was largely completed in the southern half of Britain by AD 87, when the Stanegate was established. The conquered territory became the Roman province of Britannia. Attempts to conquer northern Britain (Caledonia) in the following centuries were not successful.