What made the Roman Empire an empire, a global world and not just a juxtaposition of cities or provinces? This question has rarely been raised. Most of the studies have concerned local (civic or provincial) stories or have been dominated by the center-periphery model , developed with perfection by Fergus Millar's The Emperor in the Roman World. Here, I would like to challenge this model and raise the issue of the internal dynamic of the empire and of the awareness people had of this dynamic, especially in the period between the second and fourth centuries. There are several ways to address this issue: through analysis of the way space is perceived and represented in geographic works; through study of the places of contact and the networks that linked the diff erent parts of the Mediterranean; or through the study of mobility, which will be my point of view here. From the second century CE onward, the Roman Empire was seen as a fl uid space where people were free to move, and this freedom was regarded as an important innovation in human history. What made this fl uidity possible and what were its eff ects? Like other Mediterranean societies from the Bronze Age onward, the Roman Empire was a world in motion. This does not mean that movement was the same during all these periods or that everybody moved: to be considered mobile, a society needs only a certain rate of mobility. Neither does it mean that the authorities always considered this phenomenon in the same way. Indeed, the study of laws about mobility in diff erent periods and circumstances 9781107032668c06_p130-152.indd 130 9781107032668c06_p130-152.indd 130