Groundwater theme is a component of Encyclopedia of Water Sciences, Engineering and Technology Resources in the global Encyclopedia of Life Support Systems (EOLSS), which is an integrated compendium of twenty one Encyclopedias. Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. This theme presents a perspective of the field of groundwater and an overview of the important aspects of the subject such as, natural origin and distribution, characteristics under diverse climates and surrounding rocky environments, exploration and management, natural quality and human related sources of contamination, sustainable exploitation of resources, protection and current research trends. The content of the theme on Groundwater is organized with state-of-the-art presentations covering several topics: Origin, Distribution, Formation, and Effects; Typical Hydrogeological Scenarios; Transport Processes in Groundwater; Transport Phenomena and Vulnerability of the Unsaturated Zone; Groundwater Development; Groundwater Use and Protection; Groundwater Management: An Overview of Hydro-geology, Economic Values and Principles of Management; Special Issues in Groundwater, which are then expanded into multiple subtopics, each as a chapter. These three volumes are aimed at the following five major target audiences: University and College students Educators, Professional practitioners, Research personnel and Policy analysts, Managers, and Decision makers and NGOs