Continually, urban and suburban territories break up and are recomposed, as areas of intense residential mobility. The work analyses the processes of social differentiation leading to phenomena of social and spatial specialisation of the territories, either related to impoverishment or to gentrification. The study area is varied, being urban and rural at the same time. The research which relates to the urban fringe of the Paris agglomeration focuses on the area of influence of the economic centre of Cergy Pontoise, in order to reduce the incidence of professional factors on the mobility. As well as the residential mobility, the strategies and tactics of the households, acting as "micro-actors", are examined. This justified the use of regular statistical data, but also qualitative investigations, in order to highlight the logics on which the individual choices are based. These strategies, by their small but multiple impacts, influence the social and urban composition of the territories.