This book, generated under the auspices of the Geological Society of London’s History of Geology and Hydrogeological Groups, contains 20 papers from authors in the UK, USA, Germany and Austria. Historically, it gives examples of the influence of groundwater on battlefield tactics and fortress construction; describes how groundwater was developed for water supply and overcome as an obstacle to military engineering and cross-country vehicular movement by both sides in World Wars I and II; and culminates with examples of the application of hydrogeology to site boreholes in recent conflicts, notably in Afghanistan. Examples of current research described include hydrological model development; the impact of variations in soil moisture on explosive threat detection and cross-country vehicle mobility; contamination arising from defence sites and its remediation; privatization of water supplies; and the equitable allocation of resources derived from an international transboundary aquifer.