It is difficult to believe in these times of global warming and recurrent drought that the Earth is essentially a glacial planet. Over millennia there have been numerous Ice Ages, the latest of which started to end a mere 20 000 years ago. It takes a global temperature decrease of only 6 °C to revert large expanses of central Europe and Asia to permafrost. Ten percent of the Earth’s surface some 15 million km² is currently covered by glaciers. There are glacierized regions or mountainous icecaps in every continent of the world, even on the equator. What use glaciers? These great, implacable frozen masses of ice and snow remind us of our place in time and space, and have valuable uses, vital to the daily lives of millions of people. The information they have trapped in their frozen interiors may also be critical to human development and survival.Seventy-five percent of the world’s freshwater is stored in glaciers, and the water they release is used to produce hydropower and for irrigation. Glaciology the scientific study of snow and ice-is catalysing the development of early-warning systems for some glacier-related natural disasters such as ice avalanches. Furthermore, glaciers provide a frozen record of atmospheric conditions over long periods of time. Ice cores from glaciers have been used to establish, for example, the levels of carbon dioxide and other chemicals in the atmosphere in pre--industrial times. Finally, glaciers can provide indications of climatic change in the past and of changes occurring at present.The retreat of glaciers worldwide during the past century is evidence of global climate change that can be easily understood by everyone, and it is important that we recognize, document and study this important signal of climate change. Systematic worldwide observation of glaciers started in 1894. In 1976 a worldwide inventory of glaciers and ice sheets was begun, the first report of which World Glacier Inventory: status 1988 was published in 1989. Glacier monitoring is now coordinated by the World Glacier Monitoring Service, established in 1986 with help from UNEP and other national and international bodies.