The proposed book deals with the role of changing groundwater base level on the adjacent hydrological systems. It summarizes, compiles and compares results of current and paleo base levels, using examples from all over the world.A classification is given for marine or continental groundwater base levels with special attention to those below sea level. The factors controlling base level changes and the methods for their determination are elaborated. Holocene and future changes are discussed with their effect on salinization and flushing mechanisms of groundwater. All topics described in the book are accompanied by examples and references from all over the world.