The understanding and predictability of the observed decline of Arctic sea ice depends on the knowledge of its mass balance in a warming environment. While sea ice extent is monitored by passive microwave sensors for decades, only recently its volume is measured by basin-scale sea ice thickness observations of satellite altimetry missions. The current sea ice thickness sensor is SIRAL, a synthetic aperture radar altimeter on-board ESAs CryoSat-2. Altimetry missions measure freeboard, the height of the ice surface above the local water level, which can be converted into sea ice thickness. The conversion is very sensitive to errors in the freeboard retrieval and uncertainties in input parameters for the freeboard-tothickness conversion. Snow on Arctic sea ice plays a crucial role, since its regional variable physical parameters control the penetration of the Ku-Band radar waves and snow depth is required for the sea ice thickness retrieval algorithm, but not measured Arctic-wide. We present Arctic sea ice freeboard and thickness maps from CryoSat-2. The results are compared to available airborne validation datasets, which have been collected in an international validation program (CryoVEx). Sea ice thickness datasets obtained by airborne electromagnetic inductions sounding are available in Arctic spring of 2011 and 2012 and late summer of 2012 to quantify the uncertainties of the CryoSat-2 Arctic sea ice thickness data product of the Alfred Wegener Institute. Compared to the Arctic, only few validation datasets exist for CryoSat-2 sea ice data in the southern Ocean. We present the layout and first results of CryoSat-2 validation campaigns in the Weddell and Bellingshausen Seas between June and October 2013. The complicated snow properties of Antarctic sea ice may increase the uncertainty of CryoSat-2 sea ice thickness data, however sea ice volume estimates in the southern hemisphere are needed as well to understand the contrasting increase of Antarctic sea ice area.