Groundwater is a hidden part of the water cycle due to its subterranean nature but is highly connected to rivers, lakes, and wetlands. In this chapter, we review the physical basis of aquifers (groundwater reservoirs), their hydrodynamics, and hydrogeological parameters (porosity and permeability) that collectively define different types of aquifers. We explore the relationships between groundwater and surface water and define how aquifers function in terms of (1) groundwater flow and transport of solutes and particulate matter; (2) groundwater age, which affects ecosystem processes, physical and biological reactions, and groundwater resources and (3) modeling of the above processes. We also consider the chemical composition of groundwater and the origin of compounds and water-rock interactions that influence water quality. Finally, we discuss chemical and nutrient fluxes in aquifers and biogeochemical reactions, with a focus on oxygen and nitrogen.