This fully illustrated study examines and compares the roles of the US Navy submarines and the Imperial Japanese Navy's anti-submarine warfare capabilities during World War II. The Imperial Japanese Navy (IJN) began the war by assigning a very low priority to antisubmarine warfare (ASW). Although Japan heavily depended on imports to feed its population and war industries, the IJN's leadership thought the war would be over before shipping losses to US Navy (USN) submarines began to hurt. The early operations of USN submarines seemed to confirm these optimistic projections: in 1941-42, USN submarine operations were largely ineffective. Several factors were in play, including the faulty Mark XIV torpedo, conservative tactics, and overly conservative submarine captains. Eventually, though, the older and ineffective captains were weeded out, and the new generation of wartime submarine commanders proved aggressive and innovative. When these qualities were combined with reliable torpedoes, more Gato-class boats, superb intelligence on Japanese naval and shipping movements, and the development of advanced radar suitable for employment on submarines, the results were devastating. Losses to American submarines reached crippling proportions by 1944. The low priority accorded to ASW by the IJN resulted in a lack of ASW escorts and modern weaponry, and an inability to develop tactics. This superbly illustrated study explores these factors, and the role that US submarines played in supporting all the major fleet operations in the Pacific Theater, knotching up almost 500 patrols by war's end for the loss of 52 submarines to Japanese anti-submarine capabilities. The technical and tactical developments implemented by the opposing sides are documented in detail, including improvements to US sub design and weaponry and more aggressive tactics, and the Japanese development of destroyer escorts, changes to depth charge design, and improved submarine detection capacity.