Historians have paid little attention to the British Army’s experience with signals intelligence during the First World War. However, this was one of the Army’s most important sources of information about crucial matters such as the order of battle and intentions of the German and Turkish Armies. While efforts to weed the records on these topics have created major gaps in the sources, surviving evidence throws an entirely new light on the British Army in the Great War. It allows one to trace the evolution of the Army’s signals intelligence organisations, to determine many of their successes and failures, to show how this intelligence affected various operations, and, indeed, to demonstrate that signals intelligence influenced the operations of the British Army as much as those of the Royal Navy. The material reproduced in this volume includes excerpts from reports by Army Headquarters in France, Italy and Mesopotamia and the Military Intelligence Directorate.