The emergence of operationally effective submarines in the decade or so preceding the outbreak of World War I revolutionized naval warfare. The pace of change in naval technologies generally in the late nineteenth century was unprecedented, but the submarine represented a true revolution in the nature of war at sea, comparable only to the emergence of naval aviation in the period following the First World War or of ballistic missiles and the atomic bomb following the Second. It is therefore not altogether surprising that the full promise and threat of this novel weapon were not immediately apparent to observers at the time. Even after submarines had proved their effectiveness in the early months of the war, navies were slow to react to the new strategic and operational environment created by them. The Royal Navy in particular failed to foresee the vulnerability of British maritime commerce to the German U-boat, especially after the Germans determined on a campaign of unrestricted submarine warfare-attack without warning on neutral as well as enemy merchant shipping-in 1917. In Defeating the U-boat: Inventing Antisubmarine Warfare, Newport Paper 36, Jan S. Breemer tells the story of the British response to the German submarine threat. His account of Germany's "asymmetric" challenge (to use the contemporary term) to Britain's naval mastery holds important lessons for the United States today, the U.S. Navy in particular. The Royal Navy's obstinate refusal to consider seriously the option of convoying merchant vessels, which turned out to be the key to the solution of the Uboat problem, demonstrates the extent to which professional military cultures can thwart technical and operational innovation even in circumstances of existential threat. Although historical controversy continues to cloud this issue, Breemer concludes that the convoying option was embraced by the Royal Navy only under the pressure of civilian authority. Breemer ends his lively and informative study with some general reflections on military innovation and the requirements for fostering it.