This book complements the Geological Society’s Special Publication 362: Military Aspects of Hydrogeology. Generated under the auspices of the Society’s History of Geology and Engineering Groups, it contains papers from authors in the UK, USA, Germany and Austria. Substantial papers describe some innovative engineering activities, influenced by geology, undertaken by the armed forces of the opposing nations in World War I. These activities were reactivated and developed in World War II. Examples include trenching from World War I, tunnelling and quarrying from both wars, and the use of geologists to aid German coastal fortification and Allied aerial photographic interpretation in World War II. The extensive introduction and other chapters reveal that ‘military geology’ has a longer history. These chapters relate to pre-twentieth century coastal fortification in the UK and the USA; conflict in the American Civil War; long-term ‘going’ assessments for German forces; tunnel repair after wartime route denial in Hong Kong; and tunnel detection after recent insurgent improvisation in Iraq.