Abstract The hydrogeology of carbonate rocks is more variable than that of any other major rock type. Whereas some carbonate formations are among the most permeable and productive aquifers known, others have such low permeability that they serve as major confining layers and yield virtually no water to wells. In carbonate terranes throughout North America and the world, hydrogeologic variability generally is the rule rather than the exception. This wide range of hydrogeologic behavior results from the complex interaction of many geologic and hydrologic variables. Most important of these variables is the continually evolving porosity and permeability distribution of the rock mass, which is a result of the chemical interaction of the carbonate rocks and the water moving through them. The flow of water through the rocks is essential for aquifer development, for it is flow that serves as the primary transport mechanism in the dissolution of carbonate rocks. The purpose of this chapter is to describe the major hydrogeologic controls, principles, and processes that affect the occurrence, movement, storage, and geochemistry of ground water in carbonate rocks. The chapter represents a synthesis of selected concepts and theories currently thought to explain some of the major similarities and differences of carbonate rocks in various settings. It should be emphasized, however, that many ideas in this chapter are not universally accepted as fact. Even among the authors of this chapter there are several major points of disagreement. Generalizations about complex areas have been incorporated to demonstrate major principles. While these simplifications may be valid on a regional basis, they may not accurately define localized areas within a region. For details that concern specific regions, see chapters 3, 19, and 23 through 29 of this volume.