This textbook provides a complete introduction to Hydrogeology. It is a comprehensive reference for earth science professionals involved in groundwater exploitation as well as for geotechnical engineers. This English translation of the German textbook "Hydrogeologie" by Hölting & Coldewey, which has been published in its 8^th edition, provides insights into the sources and reservoirs of groundwater, the dynamics of fluid flow, and the physical and chemical composition of groundwater. It also gives an overview about the economic value of groundwater and its exploitation and use. A consistent use of the internationally accepted SI units as well as the formula symbols in the text contributes to the understandability.