Most of the world’s mountains are rich in water and, as such, play a pivotal role in the global water cycle. They provide water for diverse human uses and ecosystems. Growing water demands as well as climate change will lead to ever-increasing pressure on mountain waters. Overcoming water-use conflicts and maintaining the ecological functioning of mountain waters presents a highly challenging task and is indispensable for sustainable development. This book extensively portrays the highly diverse attributes of mountain waters and demonstrates their paramount importance for ecological and societal development. The extensive summaries on the scientific basics of mountain waters are supplemented with considerations on the diverse water uses, needs for management actions, and challenges regarding sustainable water management. This overview concerns not only the mountain areas themselves but also downriver reaches and their surrounding lowlands, and, therefore, the relationship between mountain and lowland water issues.