An acclaimed military historian examines the vital role of British naval intelligence from the mid-nineteenth century to the end of the Cold War. In this comprehensive account, Andrew Boyd brings a critical new dimension to our understanding of British naval intelligence. From the capture of Napoleons signal codes to the satellite-based systems of the Cold War era, he provides a coherent and reliable overview while setting his subject in the larger context of the British state. It is a fascinating study of how naval needs and personalities shaped the British intelligence community that exists today. Boyd explains why and how intelligence was collected and assesses its real impact on policy and operations. Though he confirms that naval intelligence was critical to Britains victory in both World Wars, he significantly reappraises its role in each. He reveals that coverage of Germany before 1914 and of the three Axis powers in the interwar period was more comprehensive and effective than previously suggested; and while British power declined rapidly after 1945, the book shows how intelligence helped the Royal Navy to remain a significant global force for the rest of the twentieth century.