Thermokarst denotes the processes, landforms and sediments associated with ablation usually by thawing of excess ice in permafrost. Thaw has two important geomorphic consequences: (1) a reduction in soil strength due to the change to an unfrozen state, and (2) a reduction in soil volume (consolidation) due to the loss of excess ice. Both factors promote geomorphic and sedimentary processes that can transform the morphology of the land surface and the physical properties of the substrate. Because thermokarst activity is usually initiated by disturbances to the energy balance at or near the ground surface, thermokarst phenomena are sensitive indicators of environmental change. This chapter reviews the processes, development, activity and phenomena associated with thermokarst in permafrost soils, before considering the relationship between thermokarst and global warming. Thermokarst activity in frost-susceptible bedrock is discussed by Murton et al. (2006).