a. Aim This work introduces the hydrology of groundwater systems. Focusing on natural groundwater systems, this work is meant to get students of environmental, biological, or engineering sciences started with hydrogeology. b. Main concepts and methods covered The most relevant hydrogeological technical terms are introduced and defined. In separate sections, the most important processes of groundwater systems, groundwater recharge, groundwater storage and groundwater discharge, are explained and illustrated. Conceptual hydrogeological models are presented as valuable tools to combine the available information about these processes and their underlying physical structure into visual hypotheses of the groundwater system to prepare for discussion among colleagues and to prepare for the application of groundwater models. c. Conclusion/Outlook The introduction to groundwater systems and definitions given in this chapter will enable students to better understand groundwater systems and their interactions with other environmental and anthropogenic systems.