Dams are a major source of electricity globally, with hydropower generating 16 percent of the world’s total electricity and 71 percent of all renewable electricity in 2016. Many developing countries possess great untapped hydropower potential. Sub-Saharan Africa, for example, is estimated to have tapped less than 8 percent of its hydropower potential. Proponents of dams praise them as a source of low-carbon electricity, estimated to reduce annual emissions by about 2.8 billion tons of carbon dioxide equivalent. Dams also provide wide-ranging benefits in terms of flood control, irrigation, navigation, and job creation. But harnessing the power of the river comes with concentrated costs, from fragmenting the river system and destroying natural habitat to triggering ecological hazards and displacing millions of people. As the world is undergoing an energy system transformation toward renewable sources to combat climate change and meet emission reduction targets outlined in the Paris Agreement, understanding the costs and benefits of dam construction has important policy implications. In this project, the authors compiled a global geospatial database of dams, the Global Dam Tracker (GDAT), to enable rigorous research on the costs and benefits of dam construction. The project was motivated by the absence of a comprehensive, reliable, real-time, easy-to-use database on global dam construction. Such data could allow policymakers to make informed decisions on the use of hydroelectric power in the future, based on systematic evaluations of the costs and benefits of hydroelectric dams along the dimensions of energy access, climate change mitigation, water supply, ecological preservation, and population displacement.