The Trail Smelter Case arbitration is famous in international environmental law. Following a decision by an international tribunal in 1941, Canada agreed to pay indemnities for air pollution caused by a Canadian plant in the state of Washington. This case illustrates a problem of utmost importance for humankind, namely transnational vulnerabilities. The author argues that historians of technology can contribute to a deeper understanding of these problems, which are often intimately linked with technology. He cites two ongoing projects that study such vulnerabilities. The Eurocommons project focuses on Europe's common skies, rivers and seas and studies conflicts concerning the exploitation of their resources, while the Eurocrit project analyzes how transnational infrastructures have cre! ated new forms of interdependencies and shared vulnerabilities! among European nations. The author argues that cosmopolitan scholarship, a close cooperation among scholars from many countries with intimate knowledge of different contexts, is necessary for studying transnational problems.