In this paper I propose three hypotheses. First, I assume that the struggle within illegality presents some features that influence the result of the mobilization and the treatment by public actors. Some actors without any legal resources can use "negative resources" by disturbing public order (mass challenge) and get some considerations from public actors: welfare and houses. Second, I assume that the level of institutionalization and of social, economic and political capital of the dwellers impacts on the mode of action used. I will show that some squatters have legal resources and use it in order to conserve alternative spaces in the city. But most of slum inhabitants are not able to develop a large network of resources. Third, I assume that the radicalism of such occupations drives decision makers and officials to implement specific urban policies at the local level. In the first part of the paper, I present my methodological approach and the databases. Second, I present a theoretical point of view of what is an illegal occupation. Finally, I address the conditions of mobilization within illegality and the different configurations of squats and slums in the Ile-de-France Region