The removal of ice by melting at the base of a glacier. At the base of grounded temperate ice, melting is either fuelled by the geothermal heat flux and the conversion of the kinetic energy of basal sliding to heat, or results from variations of the pressure-melting point. Pressure melting, however, tends to be balanced by regelation. Typical continental geothermal heat fluxes G of 0.050.15 W m-2 imply potential basal ablation G/Lf of 514 mm w. E. A-1, where Lf is the latent heat of fusion. Much greater geothermal heat fluxes are found in areas of Active volcanism. If all of the energy of basal sliding is converted to heat, basal ablation ubb/Lf at rates of 330 mm w. E. A-1 is implied by sliding velocities ub of 10100 m a-1 and basal shear stress b of 105 Pa. Basal ablation rates tens or hundreds of times greater are implied beneath ice streams. At the base of an ice shelf or floating tongue, melting occurs because of convection of warmer sea water to the ice-water interface, supplying the required latent heat of fusion. The rate of melting depends on the temperature of the sea water and the efficiency of the heat transfer between the seawater and the base of the ice shelf. Basal ablation rates beneath ice shelves or floating tongues can reach tens of m w. E. A-1, equivalent to heat transfer at hundreds of W m-2.