Gravitational waves represent today a hot topic, which promises to play a central role in astrophysics, cosmology and theoretical physics. Technological developments have led us to the brink of their direct observation, which could become a reality in the coming years. The direct observation of gravitational waves will open an entirely new field: gravitational wave astronomy. This is expected to bring a revolution in our knowledge of the universe by allowing the observation of hitherto unseen phenomena such as coalescence of compact objects (neutron stars and black holes), fall of stars into supermassive black holes, stellar core collapses, big-bang relics and the new and unexpected. This book provides a comprehensive review of gravitational waves. It includes a wide range of contributions by leading scientists in the field. Topics covered are: basics of GW with some recent advanced topics, GW detectors, astrophysics of GW sources, numerical applications, and several recent theoretical evelopments.