Starting from the observation of the paucity of comparisons between Northern and Southern squatting, this chapter analyzes the obstacles to such comparisons and tries to open pathways to develop a comparative research agenda beyond regional studies. Squatted settlements are bigger in the South, squatting policies are much more institutionalized in the South, forms of collective action diverge, and local contexts are extremely different. However, we believe that conceptual, methodological and contextual problems can be overcome by adopting a political economy approach emphasizing toleration and formalization policies as a common point of departure in order to address the puzzling question of persistence of illegal housing in both hemispheres. We identify three main reasons that explain the persistence of squats and slums in the 21 st century, not only due to policy failures. Firstly, public administrations contribute to create illegality by planning the city for growth and attractiveness. Secondly, they strategically tolerate illegal housing to defend economic and political interests. Finally, both Northern and Southern squatters collectively organize to resist evictions and to survive in cities in times of crisis. Behind these common results, we argue that each research tradition can learn from the other and their synthesis mutually contributes to the improvement of our knowledge of illegality, informal housing and urban policies.