Although less prominent now than in the recent geological past, glaciers are nevertheless common in the colder areas of the globe. Estimates vary, but there is general agreement that there are in excess of 100,000 glaciers around the world, which tie up some 2% of the world's water supply. 

Generally restricted to higher latitudes and altitudes, glaciers are masses of ice in downhill motion. As snow accumulates into snowfields, it remelts and recrystallizes into ice. In sufficient thickness, the ice begins to flow downhill. Ultimately, the glacier reaches warmer temperatures and the ice melts. The balance between the amount of ice added (accumulation) and the amount lost (ablation) by evaporation (sublimation) and melting is called the regimen of a glacier. If the regimen is positive (more is added than is removed) the glacier grows and advances. If the regimen is negative, the glacier shrinks and if the regimen is balanced, the glacier stagnates. Even while the glacier as a whole may stagnate or retreat, the ice is always moving downhill. 

There are two basic types of glaciers. Most people are familiar with valley or alpine glaciers. Larger glaciers, mantling entire regions, are called continental glaciers. Although continental glaciers repeatedly covered large areas of Europe and North America in the past, today they are restricted to Greenland and Antarctica.