Until recently, the mass balance of the Antarctic ice sheet was not well known. Here, I review recent progress in determining its magnitude and temporal evolution, the physical processes that control the observed changes in ice sheet mass balance, and the important knowledge gaps that remain. The results highlight that the linkage between climate change and the Antarctic ice sheet is more complex than anticipated and that major observational and numerical modeling advances will be needed before we can reliably predict its evolution in a warming climate. At present, the Antarctic ice sheet is losing mass at a rate almost comparable to that of the Greenland ice sheet, about 250 +/- 31 Gt/year or 0.7 mm/year sea level rise, and the mass loss is increasing with time, at a rate slightly below that observed in Greenland, at 14 +/- 2Gt/yr(2). The Antarctic ice sheet is therefore a major contributor to sea level rise and its contribution is slowly increasing with time. (C) 2011 John Wiley & Sons, Ltd. WIREs Clim Change 2011 2 324-331 DOI:10.1002/wcc.110