Abstract Caves are naturally occurring underground voids. They occur in many types of rocks in many ecosystems. Common types of caves include solutional (karst) caves, lava tubes, sea caves, talus caves, regolith caves (formed by soil piping) and glacier caves (ice-walled caves). Caves range in size from only a few meters long to over 100 km long; the longest known cave, Mammoth Cave in Kentucky, is 580 km long. They also vary considerably in complexity, depth, volume, number of entrances, and many other fundamental parameters. A concept closely related to solutional caves is karst. Karst is a landscape that forms through the dissolution of soluble rock. The most common soluble rock types are limestone and dolomite. Other types of soluble rock that form karst include marble, various evaporates (e.g., gypsum, anhydrite, and halite), and occasionally sandstone or quartzite. Karst landscapes are characterized by internal drainage, losing streams, sinkholes, caves, and springs. Karst is a prevalent landscape; ~20% of the United States is karst. Veni et al. (2001) provides a good introduction to karst. Books such as Moore and Sullivan (1997) and Gillieson (1996) provide somewhat more detailed, but still easily understood discussions of the subject. Caves are significant, non-renewable, geological resources. They are significant in their own right and also because they house other significant resources including geological resources (mineral deposits, paleontological remains, etc), biological resources (cave adapted and/or dependent microbiology, invertebrates, and vertebrates), and cultural resources (archaeological, historical, religious, and cultural materials, remains, and