Over broad regions of the earth, such as stable continental interiors and deep ocean basins, isostatic equilibrium is a good approximation. The isostatic requirement relates the density distributions in the continental and oceanic lithospheres. These density distributions are the result of differences in both composition and temperature. In regions of isostatic equilibrium the dipole moment of the near-surface density distribution is proportional to the local geoid anomaly. By using accepted crustal density distributions and either error function or linear temperature distributions the difference in geoid height between stable continental areas and deep ocean basins has been determined as a function of the continental lithospheric thickness. If the continental lithosphere were greater than 200 km thick, the geoid anomaly over the continents would be systematically negative in relation to that over the ocean basins. By using the GEM 9 satellite geoid the mean geoid anomalies over ocean basins and stable continental areas have been obtained. No systematic difference between continental and oceanic geoids is observed. We conclude from our geoid studies that the thickness of the continental lithosphere is near 180 km. This is in good agreement with various interpretations of the surface heat flow observations.