The Cassi were one of five Celtic 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.
- Julius Caesar, Commentarii de Bello Gallico 5.21
- Cunliffe, Barry W. (2005). Iron Age Communities in Britain: An Account of England, Scotland and Wales from the Seventh Century BC until the Roman Conquest. Routledge. p. 139. ISBN 978-0-415-34779-2.
- Webster, Graham (2000). Boudica: The British Revolt Against Rome AD 60. Routledge. p. 32. ISBN 978-0-415-22606-6.
- Williamson, Tom (2000). The Origins of Hertfordshire. Manchester University Press. pp. 43–44. ISBN 978-0-7190-4491-5.
- Dyer, James (2002). Ancient Britain. Routledge. p. 168. ISBN 978-0-415-15151-1.