The present publication on karst hydrology and physical speleology combines two subjects which have up to now been treated separately. The two fields of knowledge have gone their separate ways, less as a result of differences in subject matter than of varying approaches. The focal point in karst hydrology lies in the description of subter ranean water with its physical and chemical properties, whereas physical speleology de scribes subterranean cavities with their contents (air, water, and sediments), which gen erally have been created by water. Such cavities can be correctly interpreted only by means of a knowledge of karst hydrology, yet they in turn yield indications of the pro perties of karst water. Karst hydrology and physical speleology are thus two aspects of the subterranean karst phenomenon and should be viewed congruently. This book addresses geologists, hydrologists, geomorphologists, geographers, and karstologists, above all speleologists, as well as all friends of caves, especially the cavers among them. Its contents must therefore appeal to two groups: on one hand to the academically trained, whether university faculty, graduates, or students, who as a rule have the necessary basic knowledge to be able to understand the theoretical com ments; on the other hand to the laymen, who have fust-hand experience from their own observations in caves, but who often do not dispose over the scientific foundation necessary for an understanding of the phenomena. Therefore occasionally more attention will be given to problems of a simpler nature and to questions of technical terminology.