'Groundwater Theory' has emerged from discussions about methodology for groundwater in environmental projects, an inconsistent mixture of analytical and empirical formulas and vague field approaches, mostly based on traditional field methods in hydrogeology. Theoretical basics were revised, not based on field observations, but on their mathematical and physical foundations, as analytical calculation is about solving a differential equation that describes a potential gradient as a continuum. The book reassesses the significance of hydrogeological equations and the concise meaning of terms used therein. Existing analytical equations are improved. The gap between steady-state and time-dependent calculations could be bridged. Myths are debunked. The book challenges traditional consensus on groundwater flow and approaches the theory of groundwater from a new perspective. The book provides new insights that might interest professionals, lecturers and students in hydrogeology and anyone involved with groundwater related environmental aspects.