Active ice streams move one to two orders of magnitude faster than the ice bounding them, resulting in heavily crevassed shear margins. The high velocity of ice streams is attributed to lubrication by water at the glacier bed. Some shear margins are stabilized by valley walls, but others appear in places with no obvious underlying topographic control. The latter may be unstable against perturbations in the speed of the ice stream or in the influx of ice from outside the margin. Changes in basal hydrology can result in changes in ice stream width and velocity, and may result in shutting down the flow. Ice streams commonly transition into ice shelves in grounding zones, a few kilometers wide, in which coupling of the ice with the bed gradually diminishes. The stress and velocity distribution in ice shelves differs from that in grounded glaciers owing to the lack of any traction on the bed. The mass balance of ice shelves is affected by melting of the shelf base by sea water circulating beneath the shelf. Ice shelves can disintegrate in a matter of days or weeks when atmospheric temperatures are warm enough to result in appreciable melting on the shelf surface.