Guam is a rapidly developing island with over 70% of its water supply coming from the carbonate Northern Guam Lens Aquifer (NGLA). Identifying, mapping and interpreting karst features of the NGLA is crucial to its successful development and protection. This project is a detailed inventory, interpretation, and discussion of karst features of the NGLA and other limestone areas of Guam. Guam exhibits characteristic island karst features resulting from interaction of marine and fresh ground water as well as numerous classic continental karst features. The categoriesof Guam's karst features are: karren and phytokarst, epikarst, surface flow landforms, closed contour depressions, caves, and springs. Karren and phytokarst are diverse. The epikarst appears identical to the epikarst of other carbonate islands. Closed depressions on Guam are dissolutional, constructional, and human-modified. Dissolutional closed depressions include large cockpit karst sinkholes, point recharge sinkholes, collapse sinkholes, and blind valleys. The largest closed depressions are probably constructional. Many depressions have been modified to act as ponding basins. The main categories of caves on Guam are pit caves, stream caves and flank margin caves. Numerous pit caves vary widely in size and reach depths up to 50 meters. Stream caves are associated with allogenic rainwater catchment by volcanic rocks. Flank margin caves, formed along margin of the fresh-water lens, are exposed on the cliffs in Northern Guam and indicate previous sea-level still stands. Additional types of caves found include fracture caves and voids created on the top, bottom and within the freshwater lens. Springs discharge freshwater at the coastline.