This is the first comprehensive account of the Imperial German Navy’s codebreaking organisation during the First World War, and it addresses what the dean of cryptologic historians, David Kahn, referred to as ‘one of the greatest missing links in cryptologic history’. The author had access to extensive unpublished archival material, and he challenges the existing historical narrative, arguing that the German decryption organisation, known as the Entzifferungs-Dienst, or ‘E-Dienst’, was much more successful than historians have been willing to admit. The creation and operations of the German decryption service took place in the midst of a revolution in intelligence, which also featured the creation of the well-known British organisation, Room 40, whose history has far eclipsed the German. The British Admiralty’s acquisition of all three German naval codebooks in the first months of the war gave them a tremendous advantage in the information war, yet the author reveals that without the benefit of any captured codebooks, the E-Dienst—centred in the Neumünster wireless station near Kiel—was still able to break most of the main codes and ciphers of the Grand Fleet and maintained that ability well into 1917\. The author shows that the decryption, assessment and reporting architecture and processes of the E-Dienst were especially well-suited for supporting the U-Boat commerce war, yet proved less effective in providing actionable intelligence support to the fleet commander during sorties and deployments of the surface fleet. The author explains that the E-Dienst was hobbled by the Imperial Navy’s divided command structure, which hindered cooperation in the face of inadequate resources, and by the lack of understanding by the navy’s senior officers of the strategic value of radio intelligence. Ultimately, the Imperial Navy’s failure to recognize or act on suspicions that its encrypted signals were being decrypted and read by the Royal Navy completely undermined its ability to maintain the security of its own encryption. Despite these handicaps, the author shows that the E-Dienst achieved an extraordinary level of success. This compelling book makes a unique and invaluable contribution to the history of the intelligence campaign during the First World War.