Radical and autonomous urban movements like the European squatters' movement tend to resist integration into the institutions of the state, although particular legal and political conditions in each country or city may significantly alter this tendency. In this article, I examine the controversial issue of ‘institutionalization’ among squatters, focusing on the few cases of legalized squats (social centres) in the city of Madrid. Negotiations with the state authorities and processes of legalization are the major forms of institutionalization involving squatters. However, an anomalous kind of institutionalization also emerges once squats, whether legalized or not, become consolidated and socially accepted. For squatting to have a successful impact, then, depends on both the type of autonomy achieved by squatters and the different outcomes of the processes of institutionalization. The case of Madrid provides empirical evidence that: (1) negotiations with state authorities were very frequent among squatters, but most were defensive; (2) the few cases of legalization were due to specific conditions such as the urban centrality of the squats, single-issue identities, social network solidarity, favourable media coverage, formal organizations working as facilitators and the squatters' leadership of the process. Furthermore, legalized squats in Madrid preserved a high degree of autonomy, self-management and ties to other radical social movements. In conclusion, both the legalized squats and the squatters' movement in Madrid as a whole, avoided ‘terminal institutionalization’ and, instead, gave shape to a ‘flexible’ one.