The Hawaiian Islands are of volcanic origin but recent fieldwork has shown that sizeable karsts have developed on "beachrock" and "caprock"- local terms for emerged carbonate reefs, aeolianite dunes and calcific beach debris. The largest is the Ewa Plain in southwest Oahu. Best studied is the Moiliili Karst in the university district of Honolulu. Here, catastrophic dewatering has caused extensive damage, and a remnant of a sizeable conduit cave is easily visited. A cenote system is present on the island of Kauai. Most of the karsts have been moderately or severely impacted by urbanization and/or agricultural development but a surprising variety of surface and subsurface forms remains. Small flank margin caves in the Waianae Karst of Oahu contain intermittent anchialine pools, and a sizeable phreatic cave in Oahu's Keawaula Karst extends below the water table. Some carbonate areas unfortunately are not open to fieldwork at present, especially in military reservations and some private lands. This is a particular problem in parts of the Ewa Plain, the Kaneohe area and part of the Kahuku Karst of Oahu, and the "closed" island of Niihau. The archeologcal literature provides useful information on some of these.