Analysis of Global Positioning System (GPS) data demonstrates that ongoing three-dimensional crustal deformation in Fennoscandia is dominated by glacial isostatic adjustment. Our comparison of these CPS observations with numerical predictions yields an Earth model that satisfies independent geologic constraints and bounds both the average viscosity in the upper mantle (5 x 10(20) to 1 X 10(21) pascal seconds) and the elastic thickness of the Lithosphere (90 to 170 kilometers), We combined GPS-derived radial motions with Fennoscandian tide gauge records to estimate a regional sea surface rise of 2.1 +/- 0.3 mm/year. furthermore, ongoing horizontal tectonic motions greater than similar to1 mm/year are ruled out on the basis of the CPS-derived three-dimensional crustal velocity field.