This article assesses recent efforts in a multiethnic town in the Guatemalan highlands to address wastewater pollution, which threatens public health and tourism, the basis of the town's economy. Reporting on an ongoing program of action research, the authors trace the erosion of traditional, Mayan civic and religious institutions that were previously responsible for maintenance of a collective waterworks infrastructure, which in recent years has become the conduit for untreated sewage. They detail how a wastewater treatment plant was built in the town with external expertise and finance, and with little regard for its social and institutional sustainability, and they analyze how local government, business associations, and nongovernmental organizations are now taking steps to address these shortcomings. Treating community not as a fixed social unit but as a network of social interactions that are continually remade, the authors argue that while state and market forces have undermined traditional institutions of local governance, they may also become foci for reasserting community and rebuilding the relationships of shared responsibility necessary to manage the commons. The challenge for development practitioners working in public infrastructure and other domains is to integrate project planning and implementation into processes of community building.