The urbanism of the cities in Northern Gaul has been analysed by the way of chronological and geographical studies. In a first part, these studies given the structure to the catalogue, in which each city is classified according to the Northern Gaul's provincial division during High- and Late Empire. Moreover, second part of this research established a synthetic study of the urban character evolution of the cities during Roman Empire, from which conclusions are presented here. After Conquest, Roman's authority inherited a territory with a political and material organizations which were completely different from the other parts of Gaul because of the weak variety of native settlements. This situation favoured the birth of cities which conserved sometimes native defensive structures and/or army camps. It was especially observed along the Rhine, where cities kept dependance relationship so hard with army that they formed "military townships". During High Empire, two important poles characterized cities. Urban public and private components taken place in a common system: streetpattern. The first effect of the streetpattern was to stop the anarchic housing development and prepare the building of public monuments which given, and not justified, their chieftown title. In Gallia Belgica, the newborn cities with streetgrid seemed to be devoid of monumental ornaments. Consequently "urban settlements without monumental character" has been proposed like a new definition during their birth period. These cities are sometimes linked to army camps ("military dependant"). In Gallia Belgica and Germania Inferior cities, the monumental ornaments came later than in Germania Superior. In a first time, they were only built around political, administrative and religious centre, called "tripartite forum". Consequently, the urban project of these cities inside these two provinces were different than cities in Germania Superior, where the public buildings were concentrated in a monumental blok. At the beginning of the Late Empire, the cities have been defined by new criterias, based on High Empire urban architectural heritage. Then the defensive system became a new important preoccupation, so essential that we began to speak about "urban fortifications", instead of "city", built on ancient monumental centre. During Late Empire, the destruction of the forum (Nyon) or their transformation in military settlement (Bavay) was a consequence and a proof of the lost of their chieftown titles. Moreover, new cities seemed to be devoid of forum. At this time, Roman power lied in residential houses with more official character. Henceforth, the new architectural references were not civil but military. The function of the new buildings indicated the transformations to high medieval city: development of storage buildings, basiliques, fabricae, and finally, new episcopal centre.