A new theory of glacial isostasy has been constructed which provides, for the first time, a coherent explanation of all the phenomena associated with glacial isostatic adjustment. These include the complete set of globally-distributed and radiocarbocontrolled relative sea-level data covering the time interval from the present to 20 ka BP, the free-air gravity anomalies associated with continental regions which were once Ice-covered, the astronomically-observed non-tidal component of the acceleration of planetary rotation, and the true polar wander evident in the International Latitude Service (ILS) record of polar motion based on photo zenith-tube data taken since 1900. Taken together, these data constrain very accurately the radial variation of viscosity in the Earth’s mantle from the surface to the boundary with the molten iron core. This model of glacial isostasy, which has no free parameters, has been successfully coupled to a zonally-averaged climate model in which an active cryosphere is directly forced, through an appropriate accumulation function, by fluctuations in effective solar insolation produced by variations in the Earth’s orbit. Milankovitch experiments completed with this model show that the isostatic adjustment component is crucial in supporting long timescale oscillations on the 10⁵ a period which have been found in ¹⁸O/¹⁶O data in sedimentary cores taken from deep ocean basins.