The response of glaciers to atmospheric warming has become a key issue in scientific as well as public and even political discussions about human impacts on the climate system. The predominant tendency of continued worldwide glacier shrinkage may indeed constitute one of the clearest indications in nature of rapid climate change at a global scale. More than a century of systematic and internationally coordinated observations provide quantitative documentation of this development and a basis for model developments in view of possible future scenarios. Mountain ranges at lower latitudes have lost large percentages of their glacier areas and volumes since the end of the Little Ice Age. Many of them may even become largely to even completely de-glaciated already during the coming decades. Such changes have the potential to profoundly affect environmental conditions in and around cold mountain chains. Sea-level rise, changing seasonality in water supply, and local formation of new lakes reflect changes in the water cycle at global, continental, and regional to local scales. They are accompanied by rather marked changes in landscape appearance, slope stability, erosion/sedimentation, and hazard conditions. The monitoring of glaciers itself faces difficult challenges of vanishing glaciers with long-term mass-balance observations. Modern techniques of spatial modeling increasingly help with integrated analysis of observed phenomena and early anticipation of possible developments.