This essay focuses on squatters as historical agents in transgressing urban boundaries, thereby contributing to a reshaping of urban environments and public spaces. Bringing together research on informal housing in various urban contexts in both the Global North and South, it examines conceptually how squatters and urban activists have transgressed real property, public-private, socioeconomic and zoning boundaries, and in so doing challenged conventional notions of space and urban authority. Our approach avoids rigid definitions of informality based on illegality in favor of an analysis of boundary shifting as a result of spatial claims that are simultaneously deemed illegal while enjoying varying degrees of legitimacy due to persistence and acceptance. Adopting a historiographic and comparative perspective, the essay shows how such practices have responded to urban planning and development that produced these boundaries in the first place, often creating ever more privatized, gentrified, and securitized spaces and communities. Informal dwellers have, explicitly or implicitly, addressed the social questions underlying the use and distribution of urban space. This perspective helps to address historical processes of negotiation and contestation that have contributed to the emergence and shifting of boundaries demonstrating the malleability of dividing lines in the urban fabric. Squatters’ smallscale attempts at taking the city into their own hands therefore have the potential of challenging modes of planning and development dominated by elite technocrats and private capital interests.