In recent years, complexity science has emerged as a novel way to characterize geochemical processes in groundwater. These complex biogeochemical systems are difficult to understand because of heterogeneities at different spatial scales; highly variable reaction time scales; and nonlinear, biologically catalyzed reactions and their associated feedbacks. This book will evaluate complex systems science as a potential new approach for groundwater science and integrate concepts of complex systems science into groundwater science. New scientific opportunities and challenges whose solutions significantly could advance the predictive capability and state of science for such research will also be covered. To date, there have been recent improvements on how to understand the geochemical processes in groundwater by complexity science, and therefore it is necessary to summarize recent advancements gained by this new systematic theory, which is hoped that this can stimulate continuous efforts in advancing hydropedology and the integrated subsurface sciences. The authors of this book have multi-disciplinary background including hydrogeology, geobiology and computer science.