This dissertation asks what a socialist conception of the world looked like in the imaginations of those charged with actually building it. It focuses on the answers provided by infrastructural projects in socialist Yugoslavia—a uniquely positioned, communist-led state operating on the “other side” of the Iron Curtain. I show how the Yugoslav state charged grand spatial projects with the task of catalyzing social transformation, materializing ideology, and articulating Yugoslav socialism to domestic and international audiences. Looking at ambitious projects such as the Brotherhood and Unity Highway (1947–63) and Skopje’s reconstruction after a devastating earthquake (1963–80), I examine how the mediation of socialist existence and multiethnic/international solidarity depended upon workers’ mass participation in constructing each site as well as users’ continued engagement with them through cultural practices and everyday activation. For example, the Sutjeska Memorial Park in Bosnia-Herzegovina (1958–75; second chapter) began as a commemoration site but grew to become both a holiday locale and the finish line for a cross-republic auto-relay that hosted hundreds of Yugoslav and international racers and revelers. I argue that this continued re-imagination of the site facilitated experiences beyond those associated with the site’s official history. This dissertation makes visible how a variety of agents—architects, planners, politicians, but also everyday individuals whose lives were to be revolutionized—contoured the parameters of building both Yugoslavism and socialism. I analyze photographs, ephemera, artworks, and structures to argue that infrastructural endeavors sanctioned collective practices that operated in similar ways as the projects themselves, underscoring how the very basis of socialist existence in Yugoslavia came to depend upon an infrastructural definition of society and the self. This study aims to revitalize our understandings of the built environment’s capacity to make imaginable concrete social change and generate new ways of being and seeing.