The thickness distribution of sea ice is a result of the amount of freezing, melting, and deformation processes which an ice cover has experienced since its formation. Therefore, interpretation of ice thickness and its seasonal, interannual, and decadal variability has to take into account both atmospheric and oceanic boundary conditions, as well as ice motion and deformation, both on local and regional scales.This presentation reviews results obtained by moored and submarine upward-looking-sonar (ULS), drifting buoys, and modeling. These show a remarkable ice thickness decrease of up to 42% in most regions of the Central Arctic Ocean. Drifting buoy data and model results demonstrate that this decrease was most likely a result of changes in ice circulation regimes, and that the thinning of the Central Arctic Ocean might have been accompanied by increasing ice thickness in the marginal seas adjacent to Greenland and Canada. In turn, different circulation regimes also result in changes of divergence and the associated amount of open water, with consequences for the surface energy budget and freezing and melting. The complicated interplay between dynamics and thermodynamics will also be shown with a regional study in the Laptev Sea.Interpretation of results of regional ice thickness measurements is hampered by the lack of systematic repeated surveys. Alternative methods are required in addition to common ULS measurements, and to recent new satellite altimetry missions. Electromagnetic (EM) induction sounding has developed into a competitive technology over the past ten years. The strengths and limitations of EM measurements will be reviewed, and examples will be presented demonstrating the capabilities of airborne EM surveying for systematic ice thickness monitoring for climate studies.Recent results will be shown from the Transpolar Drift, where no thinning has been observed between 2001 and 2004, as well as from surveys between Ellesmere Island and 86°N where modal ice thicknesses ranged between 3 and 4 m in May 2004. Finally, preliminary results from the recent ISPOL drift station in the northwestern Weddell Sea will be presented. These show that ice thicknesses in the Weddell Sea are comparable to ice thicknesses in the Arctic Ocean. EM surveying has revealed the presence of at least three different ice regimes formed under different thermodynamic and dynamic boundary conditions which are clearly distinguishable by their thickness distributions. Those results also point to the importance of different snow thickness for ice thermodynamics.