This thesis investigates the phenomenon of cooperation and conflict over international water resources. In particular, it focuses on transboundary river cooperation between riparian states with asymmetrical power relations, where the dominant power is an upstream country. As argued by realist scholars, cooperation in international relations is limited. This is especially the case in regards to water resource management, when the upstream country is the regional dominant power as the asymmetric power situation makes bargaining between countries more difficult. The thesis challenges this assumption by asking the following main question: To what extent do power asymmetries prevent or inhibit cooperation between riparian states over water resources? This then raises the questions of how institutions help to mitigate competition for natural resources and how states interact in multilateral settings. To answer the central research question, the thesis develops four hypotheses and tests them by using three case studies: the Mekong, Danube and La Plata River Basins. Overall, the research argues that cooperation in transboundary river basins is possible in asymmetric power relations. It also argues that power is important, but that material power is not the only form of power that can be used by states. The research shows that, aside from the power relationship between the riparians, issue linkage and bargaining plays a central role in addition to the influence of the regional landscape where the basin is located. Further, the thesis argues that there are nuances between cooperation and conflict, and that these dynamics can change over time.