Highlighting the differences between terrestrial and underwater sensor networks, this book investigates the fundamental aspects of underwater communication. Prominent researchers consider the challenges in the development of underwater acoustic sensor networks (UW-ASNs). They introduce an effective cross-layer approach for effectively integrating communication functionalities as well as a realistic acoustic model characterized by channel utilization efficiency that enables proper setting of the optimal packet size for underwater communications. Discussing architectures for two- and three-dimensional sensor networks, the text covers the wide range of topics relating to UW-ASNs.