This comprehensive compendium overviews the complexity and uncertainty of groundwater systems, including groundwater boundaries, runoff, media, dynamic and chemical field, and stress and thermal field. The research methods and study examples were also introduced in great detail. The unique reference text is a valuable resource for researchers, academics, professionals, undergraduate and graduate students devoted to groundwater system study and complexity science. Sample Chapter(s) Preface Chapter 1: Complexity Science: From Reductionism to Holism Contents: Complexity Science: From Reductionism to Holism: Basic Concepts Main Features of Complexity Science Key Research Methods of Complexity Science Complexity Science and Earth System Science Complexity of the Groundwater System and Its Influencing Factors: Groundwater System — Open and Complex Giant System Complexity of Medium Space Complexity of Hydrodynamic Field Complexity of Hydrochemical Fields Stress Field and Thermal Field of Groundwater System Uncertainty of Groundwater System Observation and Forecast: Parameter Uncertainty Observation Scale and Uncertainty Simulation Uncertainty The Complexity of Groundwater Systems: Scientific Research Methods Characterization of Functional Complexity of the Groundwater System Theory of Dissipative Structure Characterization of Uncertainty in Groundwater System Research Examples Research Cases Based on Theoretical Analysis of Complexity Science Case Study Based on Model and Numerical Simulation Readership: Advance text for graduate students and researchers in groundwater science.