Water and energy are two fundamental elements for achieving sustainable development in Asia and the Pacific. Water and energy are inherently inter-dependent which requires a nexus perspective to address the synergy of the two sectors. However, sectoral approach has been widely used to address the two interacted issues. It is therefore important to understand the quantitative linkage of water and energy and grasp the chance to address the nexus at the planning stage of water and energy development. This study tries to provide a robust assessment methodology on water-energy nexus under the impacts of climate change up to 2050. A case study of the Ganges river basin will be conducted to help policy makers in the three countries of India, Nepal and Bangladesh to address water and energy security from the whole river basin perspective through integrated river basin management and regional cooperation.