Glaciers, ice caps and continental ice sheets cover some ten per cent of the earth’s land surface at the present time, whereas during the ice ages, they covered about three times this amount. The present ice cover corresponds to about three-quarter of the world’s total freshwater resources. If all land ice melted away, the sea level would rise by almost 65 m. Glaciers are an inherent component of the culture, landscape, and environment in high mountain and polar regions. They represent a unique source of freshwater for agricultural, industrial and domestic use, an important economic component of tourism and hydro-electric power production, yet they can also constitute a serious natural hazard. Because they are close to the melting point they react strongly to climate change, and thereby provide some of the clearest evidence of climate change and are essential variables within global climate-related monitoring programmes.