Ground water will be considered as all water of meteoric origin within the soil and underlying rocks. This brief account of the role of ground water in modifying the relief will therefore focus attention on limestone and dolomite landscapes where solution manifests itself through karst landforms. The characteristic dryness of limestone areas is a consequence of the highly permeable nature of the rock. This permeability is dependent upon the volume and interconnection of both primary and secondary interstices in the rock, and while the most permeable limestones possess both primary and secondary porosity. Underground water is able to affect topography because of its solvent capacity. The majority of investigators agree that the most important solvent in carbonate terrains is carbonic acid, which is produced by the solution of carbon dioxide from the air. Nevertheless, the importance of ground water in the evolution of other landform systems must not be under-estimated, particularly in the tropics.