This article discusses the concept of porosity and what it might offer critical urbanism. It engages recent scholarly and practical writing on the “porous city,” outlining three sets of contributions that porosity offers in analyzing contemporary urbanization patterns and in orienting planning, policymaking, and knowledge production. First, the porous city offers a critical epistemological lens focused on flow and relations, which supports mobile and infrastructural ways of viewing and knowing the city. Second, the porous city suggests the ontological features of interpenetrating geographies and temporalities, which take the urban to be a topological space of potential politics. Third, the porous city entails an ideal to which planning practice should aspire, particularly in relation to forms of urbanism and city-building that are open to multifunctionality, difference, and dynamism over time. While each of these represents a promising direction in critical urban praxis, we argue that porosity also has its limits. The porous city is conceptually malleable and normatively ambiguous and it risks overreach as well as recuperation within exclusionary and exploitative urban development agendas. We claim that the porous city should not be treated as a comprehensive global ambition, but rather, is most valuable when used to discern and build discrete architectures of power.