Water is the primary medium through which the effects of climate change will be noticed in the human environment. In consequence, impacts will also be felt in international river basins. Water balance and timing of river flow are likely to change considerably in some regions of the world. These changes are likely to alter conventional methods of transboundary water resources management. Some regulatory frameworks that govern joint management and development of international water resources might require adjustment are may be no longer applicable due to changes in underlying hydrologic circumstances. This will create legal uncertainty in relations between riparian States and can become the cause of friction. Despite the high probability that some of the world's largest river basins will undergo considerable hydrologic change over the coming decades, efforts to adjust agreements and to develop an international legal framework that provides for adequate adaptation in transboundary water basins lag behind. Water does not feature as a separate topic in international climate change negotiations; it is rarely mentioned in preparatory documents. Yet, the demand for principles and guidance on how to address climate change in transboundary water cooperation is increasing. The paper analyzes the contours of the existing legal framework; including universal rules captured in the UNFCCC and UN1997, regional efforts such as the UNECE Guidance Note and instruments adopted by the European Union, as well as the flexibility of basin treaties in dealing with hydrologic change.