Abstract: This invited contribution is intended for an audience familiar with general underwater acoustics but which wishes to gain more understanding of underwater communication using acoustic waves as a carrier. We will review some basic concepts applicable to any communication system as well as some of the diverse concepts and experimental and commercial systems used for communicating underwater. Topics considered include: acoustic transducers and arrays, some applications of underwater acoustic communications, analogue modulations, multipath, noncoherent and coherent digital transmission schemes including phase modulation, channel equalization, and state of the art. Extensive literature on the subject is provided.