Widespread water scarcity, water pollution, and depletion of freshwater resources are among the grand environmental challenges of the 21st century related to water. Central to these challenges is the fact that humanity uses too much water. But what are we using all that water for? The water footprint concept can help answer this question, and more. Addressing the relation between human freshwater consumption and water's grand environmental challenges, the water footprint concept resonates with stakeholders within and beyond the walls of science. This Primer describes the basics of the water footprint concept, how it works, and why it came about. Drawing from recent studies in the new research field of Water Footprint Assessment, it highlights some intriguing applications and delves into what is next on the exciting interdisciplinary research agenda.