Summary : Mass changes in the Antarctic and Greenland ice sheets are of considerable interest because of their sensitivity to climate change, their contributions to sea level change, and the likelihood of changes in the rates of mass loss (or gain) with climate warming. New observations of ice sheet mass balance (growth or shrinkage) primarily by satellite laser altimetry (NASA’s ICESat) and radar altimetry (ESA’s ERS) are showing details of the spatial distributions of mass losses and gains over the ice sheets, and the changes that have taken place since the 1990's. Ice sheets respond to climate change by changes in the mass input at the surface from snow accumulation (precipitation), in the mass output from melting at the margins, and the ice veloc...