What is the porous city? Porosity refers to the ambiguous zones, inbetween spaces, and thresholds that permeate urban environments. Such spaces merge into each other, providing the backdrop for the unforeseen and improvised, and blur the boundary between physical and social space. Experts in the fields of architecture, urbanism, and landscape design offer a wide-ranging critique of the modernist city and discuss the potential of porosity to enliven and illuminate a progressive urban agenda.