Presented in a clear and concise way as an introductory text and practical handbook, the 2nd Edition of this widely acclaimed book describes the basic physical phenomena governing underwater acoustical waves, propagation, reflection, target scattering and noise. It covers the general features of sonar systems, transducers and arrays, signal processing and performance evaluation. The author is an experienced scientist-engineer and teacher and he uses his personal experience to present the practical approaches to actual problems related to underwater, as well as providing an overview of current applications and explaining in a clear and concise manner the working principles of the various systems. The value of this 2nd Edition for students and teachers is maximized by the addition of well-validated exercises and problems, with detailed solutions, and a substantial appendix giving basic notions of signal processing useful for underwater acoustics. A new chapter about marine seismics features both hardware presentation and the principles for processing. Some elements about acoustical instrumentation for geotechnical surveys are included and the methods of sonar image processing are addressed, seen from the specific point of view of sonar systems, and centred upon the building of the image (eg, for seafloor mapping and water column imaging). Other engineering aspects improved upon include the presentation of fundamentals for array processing and sonar signals, as well as high-resolution methods. The technologies for measurement, especially in the field of bathymetry (interferometry), are added and the aspects related to noise are improved: much more is included about the self-noise of vessels, with some recent results, and the techniques in onboard noise reduction and control are addressed. Doppler techniques (ADCP systems), including their applications, are more widely considered. A new chapter covers marine bioacoustics and the coexistence between marine life (essentially marine mammals) and human acoustical activity, which is of more and more concern to the public. Underwater acoustical communication techniques are also more developed, stressing the underwater specificities, the applications and actual performances of today's systems.