Arctic sea ice has undergone major changes in recent years but there remains much uncertainty about its ultimate fate. Although measurements of ice extent are well established, measurements of thickness are also now key to understanding the causes and consequences of these changes. A primary objective of the CryoSat mission is to determine trends in Arctic wintertime sea-ice thickness. This is achieved by making direct measurements of sea-ice freeboard from which sea-ice thickness can be calculated. We review the current state of the art in sea-ice thickness retrieval and discuss the strategy for quantifying the uncertainties in these measurements, through the CryoSat calibration and validation activities. Finally, we plan to provide an update of the current CryoSat operations over sea ice and to present initial results should they be available.