How does informality matter in urban planning? This book examines how the concept, stemming from (urban) development discourse, has entered into the everyday language of a broad variety of urban actors in Mexico City. Informality reproduces a form of internal colonialism, translating a global discourse of “othering” into multiple micro-spatial relations between real estate developers, public planners, inhabitants and neighborhood organizations. The usual, normatively disparaging construction of informality as a practice of the urban poor is misleading and continues to marginalize communities from access to urban planning. Yet, informality matters also as an alliance-forging signifier in emancipatory struggles for housing and land. Following recent tendencies to decenter urban studies, this perspective on urbanity allows for understanding cities not as territorial units but as conflictive configurations of planning logics and cohabitation. This theoretical contribution is made on the basis of empirical research in two areas: the Historic Center and the western peripheries of Mexico City.