This report summarizes the findings of the Unimplemented Development Synthesis Workshop held at the National Socio-Environmental Synthesis Center (SESYNC) in Annapolis, Maryland from January 21–23, 2020.This report is the result of an interdisciplinary, international effort to examine the socio-environmental consequences of large-scale infrastructure projects that have been planned but not built—and reasons why projects have been cancelled or stalled. Collectively, our team examined eight cases of cancelled or stalled infrastructure projects in the Americas, drawing from our experiences in academic research and professional practice. Across cases, we found that projects were cancelled or stalled for interrelated reasons including environmental impact assessments, litigation and legislation, strong opposition and media attention, and/or increasing costs and faltering justifications. Our work further shows that cancelled or stalled development projects can create socio-environmental consequences that persist and cascade over time. The consequences of unbuilt infrastructure projects include the following: the development of new research networks; the establishment of precedents for socio-environmental assessment; the strengthening of social movements, often against the proposed development; changes in land use and land tenure; the exposure of environmental racism and injustice; and shifted development interventions. In sum, paying attention to cancelled and stalled projects provides a transdisciplinary lens for understanding broader processes of development, knowledge, power, science, and socio-environmental change. We conclude that when proposed large infrastructure projects are assessed, additional attention needs to be given to how they may shape landscapes and societies even if they are never built.This work was supported by the National Socio-Environmental Synthesis Center (SESYNC), under funding received from the National Science Foundation, Grant # DBI-1639145
