The British Army and her commonwealth Allies went to war in 1914 with little knowledge and experience of constructing permanent, shell proof protective structures. Some masonry fortifications, such as defensive blockhouses in South Africa, had been built but the Royal Engineers of the Army were more versed in simple temporary defences suitable for mobile warfare. Home defences were a limited number of forts around naval ports, and Martello Towers on the east coast. It was considered that the Navy was quite able to defend Britain's coasts. The Germans, on the other hand, as with the other continental countries such as France, Belgium, Italy, Holland, Poland, Austria, etc. had been constantly renewing and updating border forts for several centuries. They had also maintained fortification and siege elements of their armies, who were experienced in designing and constructing strong shelters. Both German and French armies began the war with a degree of expertise in what was to become a static war with little movement. However, by 1918 the British were to surpass both enemy and her allies in the design and construction, with supply and logistics, of such shell proof cover for troops and defensive positions. This book gives the history of development and innovation of concrete bunkers, pill boxes, blockhouses and general concrete constructions during the First World War. Many of these structures - some showing obvious signs of war damage - still exist in France and Belgium today. All the existing structures, with photograph (except for some which are impractical, because of dense vegetation,) are shown within. Many entries have contemporary maps showing how they fitted into a defensive system, whilst for others the location can be identified from the text. GPS coordinates are given for each entry, except for a few which are on private land and where privacy has been requested.