The Greenland ice sheet is the largest ice sheet in the Northern Hemisphere. It plays an important role in climate research because of its location in the North Atlantic, which is a region thought to play a key role in climate changes on Earth. Because snow accumulation rates in the central areas have been sufficiently high, annual layers may be resolved far back into the last glacial period. Understanding the dynamics of the Greenland ice sheet is, however, crucial for interpretation of the climate information stored in the ice. The ice sheet interacts dynamically with the atmosphere and ocean, and responds to climate changes. The present mass balance of the Greenland ice sheet is of particular interest because of its contribution to the observed sea-level rise. The Greenland ice sheet has been thought to respond gradually to climate changes through changes in its surface mass balance, but widespread areas with basal melting in the interior and fast-flowing ice streams have been identified and have challenged this view.