Karst develops on rocks where solution (or corrosion, as it is sometimes called ) is the dominant landscape-forming process, even though the full suite of other geomorphic processes occurs. All rocks dissolve in natural waters to some extent, although some dissolve much more readily than others. Of the common rocks, the most susceptible to solution are the halites (chlorides); anhydrite and gypsum (sulphates); and limestones, dolomite, and marble (carbonates). Extensive landforms on chloride and sulphate rocks are mainly restricted to arid and semi-arid terrains, because these rocks are so soluble that in more humid climates they are reduced to very low relief. On the other hand, carbonate rocks, which cover about 12% of the continental ice-free area of the Earth, are always sufficiently resistant to yield topography with characteristic solutional landforms (Figure 17.1). For this reason, this chapter will focus on New Zealand's landscapes developed on carbonate rocks. (A good review of karst on sulphate rocks can be found in Klimchouk et al. 1996.)