The Cassi are one of five tribes encountered by Julius Caesar during his second expedition to Britain in 55 BC when he crossed the Thames at Kew, and who became his allies after the Trinovantes joined him. The archaeologists Graham Webster and Barry Cunliffe both agree that nothing more is known about them, but it has been suggested that between Caesar's second invasion and the invasion of Claudius in AD 43 that the Cassi along with other tribes such as the Ancalites and Bibroci merged to form the Catuvellauni, and that Cassivellaunus may have been a member of the Cassi tribe.