Measuring changes in the mass of ice sheets has been revolutionised by the advent of satellite geodetic techniques because they provide measurements at spatial scales and at a frequency that cannot be achieved with traditional methods. Since the first publication in 1989, there have been more than 30 published estimates of ice sheet mass balance based variously on the three techniques of altimetry, gravimetry and the input-output method (e.g. Zwally et al., 1989; Wingham et al., 1998; Rignot and Kanagaratnam, 2006; Velicogna and Wahr, 2006; Zwally et al., 2011). However, the agreement between these results is poor (see figures below), and the estimates and their respective uncertainties, allow for a combined Greenland and Antarctic ice sheet mass imbalance of between -676 and + 69 Gt yr-1. Such a large spread has limited our confidence in estimates of the ice sheet contribution to sea level.