As the evidence for human induced climate change becomes clearer, so too does the realization that its effects will have impacts on numerous environmental and socio-economic systems. Mountains are recognized as very sensitive physical environments with populations whose histories and current social positions often strain their capacity to accommodate intense and rapid changes to their resource base. It is thus essential to assess the impacts of a changing climate, focusing on the quantity of water originating in mountain regions, particularly where snow and ice melt represent a large stream flow component as well as a local resource in terms of fresh-water supply, hydropower generation, or irrigation. Increasing evidence of glacier retreat, permafrost degradation and reduced mountain snowpack has been observed in many regions, thereby suggesting that climate change may seriously affect streamflow regimes. These changes could in turn threaten the availability of water resources for many environmental and economic systems, and exacerbate a range of natural hazards that would compound these impacts. As a consequence, socio-economic structures of downstream living populations would be also impacted, calling for better preparedness and strategies to avoid conflicts of interest between water-dependent economic actors. This paper is thus an introduction to the Special Issue of this journal dedicated to the European Union Seventh Framework Program (EU-FP7) project ACQWA(Assessing Climate Impacts on the Quantity and Quality of WAter), a major European network of scientists that was coordinated by the University of Geneva from 2008 to 2014. The goal of ACQWA has been to address a number of these issues and propose a range of solutions for adaptation to change and to help improve water governance in regions where quantity, seasonality, and perhaps quality of water may substantially change in coming decades