This article examines how midcentury European sociologists, planners, and architects mapped the existing city to build future communities. The neighborhood unit concept spread globally in the first half of the twentieth century. In Europe, its deployment was supported not just by modernist planning principles but by sociological abstractions of community life. Current scholarship emphasizes how these modernist principles were contested by sociologists. The present article demonstrates instead that sociological mapping was instrumental in making the concept of community legible and operable in the postwar European city. During the 1940s, mapping social relationships in urban space was increasingly thought to reveal "authentic" community life in working-class urban neighborhoods, which previously were dismissed as chaotic and promiscuous. Such new sociological mapping shaped, if often only implicitly, the planning and design of modern housing estates and New Towns across Europe and thus connected representations of bottom–up, grassroots communities to an essentially top–down planning apparatus.