Water is a fundamental resource to maintain all kinds of life on the planet. It is a resource necessary to maintain well functioning natural ecosystems, and ultimately a key resource for human survival and development. Water is needed in sufficient quantities of good quality for a variety of human uses, from food production to manufacturing to drinking and sanitation. On the other hand, excessive water surplus may cause severe damage to natural and human systems. “How much water is available at a given time in rivers or aquifers?”, “What are the minimum water requirements needed to sustain ecosystems and economic uses”, or “How can these requirements be met sustainably in a changing climate?” are fundamental questions that are addressed in this collection of articles.