Groundwater is an important water resource. The USGS collects information on the quality and quantity of groundwater and conducts advanced modeling of groundwater flow and groundwater/surface-water systems. The USGS also evaluates the effects of water-use, land-use, and climate change on groundwater, surface-water, and the ecosystems that rely on them. Groundwater is all the water below the land surface that fills the porous space in soil and rocks. The soil and rocks that store and transmit water are known as aquifers. The upper surface of the groundwater forms the water table. This web page provides an overview of USGS's research about groundwater monitoring and research. It includes related research, publications, maps, software, partners, and a collection of data and tools (Introduction).