Japan is quintessentially by geography a maritime country. Maritime surveillance capabilities – underwater, shore-based and airborne – are critical to its national defence posture. This book describes and assesses these capabilities, with particular respect to the underwater segment, about which there is little strategic analysis in publicly available literature. Since the end of the Cold War, Chinese oceanographic and navy vessels have intruded into Japanese waters with increasing frequency, not counting their activities in disputed waters such as around the Senkaku (Diaoyu) Islands and Okinotorishima where China and Japan have overlapping territorial claims. These intrusions have increasingly involved warships, including submarines, sometimes acting quite aggressively. Japan maintains an extraordinary network of undersea hydrophone arrays, connected to shore-stations which are typically equipped with electronic intelligence (ELINT) systems, for monitoring, identifying and tracking submarine and surface traffic in its internal straits and surrounding seas. Some parts of this network are operated jointly with, and are of crucial importance to, the US Navy. Japan’s superlative submarine detection capabilities would be of decisive advantage in any submarine engagement. But the relevant facilities are relatively vulnerable, which makes them very lucrative targets in any conflict. This introduces compelling escalatory dynamics, including the involvement of US forces and possible employment of nuclear options.