In September 1996, the United Nations General Assembly adopted the Comprehensive Nuclear-Test-Ban Treaty (CTBT), prohibiting nuclear explosions worldwide, in all environments. The treaty calls for a global verification system, including a network of 321 monitoring stations distributed around the globe, a data communications network, an international data centre (IDC), and on-site inspections, to verify compliance. A global hydroacoustic monitoring system is being planned and implemented for verification of the CTBT. Much of the research conducted over the past several decades on acoustic surveillance of the oceans, formerly driven by the need to detect and track submarines, is now being applied to the development of effective monitoring methods to verify compliance with the CTBT. The aim of this volume on Hydroacoustic Monitoring of the CTBT is to summarize the research being conducted in this field and to provide basic references for future research. Much of the new research emphasizes major advances in understanding the coupling of ocean acoustic waves with elastic waves in the solid Earth. Topics covered include source excitation, detection and classification of events generating hydroacoustic signals, discrimination between underwater explosions and naturally occurring events, as well as topics in coupling of acoustic to seismic wavefields.