Originally named Gesoriacum, by the 4th century Boulogne was known to the Romans as Bononia, and served as the major port connecting the rest of the empire to Britain. The emperor Claudius used this town as his base for the Roman invasion of Britain.
In Boulogne-sur-Mer in the year 1905 took place the First Esperanto Universal Congress which L. L. Zamenhof, the Esperanto creator took part.