Mountains and highlands are often called the world's natural "water towers" because they provide essential freshwater for populations both upstream and downstream. Despite this, knowledge about the significance of mountains in the hydrological cycle is still uncertain. The present article takes a regional approach, using case studies to assess and compare the hydrological significance of mountains. Methods are developed based on the experience gained in the Rhine River catchment and then applied to 19 additional selected catchments worldwide, with the river Euphrates serving as an example. The resulting comparative assessment serves as an elaboration on the hydrological significance of the world's mountains and underscores their function as sources of large, reliable, and compensatory discharge. The mean annual mountain contribution to total discharge in the river basins included in our case studies is disproportionately high, at 63%, with a mean relative mountain area of only 32%. Furthermore, distinctions can be made according to climatic regions, clearly highlighting the vital role of mountain runoff in lowlands in arid and semiarid areas. This means taking mountains and highlands more carefully into account in terms of monitoring and scientific research and especially in terms of watershed management and conflict management.