Nearly one in two French people moved house between the 1990 and 1999 census. Approximately one-third of these moves involved a change of département and over one-fifth a change of region. However, mobility is continuing to fall and is more marked over short distances (within a département). Migrations between départements, and especially between regions, are more intense at around 30 years old (age when the 1999 census was taken) than at the other ages. These migrations coincide with young people's entry into the world of work. The fact that children are leaving the parental home later is tending to set back the mobility of the under-40s. Increasingly surplus migratory balances since the 1975 census bear witness to the growing appeal of the West and South West regions (Atlantic regions). A characteristic trait of the 1990s is the renewed appeal of rural regions, aggravating the deficit in urban regions. The interregional population mix has slowed down except in the West and South West. Migrations of young adults play a key role in the population's redistribution among the regions. These migrations explain in particular the surplus (or deficit) of young children counted compared with the births recorded in each region during the period between the two censuses. A high turnover (new arrivals taking the place of departing individuals) means that only 15% of interregional movements really redistribute the population spatially. The participation of the départements in interregional trade, although highly varied within a given region, is generally fostered by the presence of a conurbation in their territory. This conurbation often plays a relay role (between the region's internal and external trade), except in highly urbanised regions where periurbanisation favours adjacent départements. Proximity, geographic complementarity and the redistribution of the Paris area's population combine to shape the main lines of the French migratory system.