Part of a series which is aimed primarily at a professional engineering or postgraduate student audience, this book concerns the basics of the propagation of sound in the sea and with the preliminary assessment via the Sonar Equations of systems performance. It includes the complexities of Normal Mode modelling of sound propagation, noise and reverberation, the subject of acoustic transduction and underwater acoustic equipment. Underwater acoustic communication which is assuming importance in scientific data gathering, communication with autonomous vehicles and sub-sea oil-field control and telemetry is treated in detail.