Terrestrial wireless sensor networks with fixed and mobile nodes were initially developed for military applications such as battlefield surveillance and are now commonplace in many industrial and civilian application areas. It is easy to appreciate that such networks could be of immense value for military and civilian underwater tasks such as surveillance, monitoring and surveying. Autonomous underwater platforms and sensor systems have already found many applications in marine research, oceanography, marine commercial operations, offshore oil industry and defense. However, due to performance limitations of available underwater communication technologies, it is often impractical to network these sensor systems and to transmit the collected data underwater. Instead, the sensor systems have to be brought to the surface before the data can be downloaded. This is not only inconvenient, but also often costly. Since the data from the sensors is unavailable in real-time, it cannot be used effectively for adaptive sensing or for real-time decision-making. In this paper, we briefly review various technologies that have been developed for underwater communications & networking. We show that there are some fundamental differences between the challenges faced by the underwater systems, as compared to their terrestrial wireless counterparts; these differences impose some limitations and open up some opportunities. It is important to recognize that while underwater and terrestrial wireless sensor networks may share some similarities, the differences between the two warrant fundamentally different solution strategies to be adopted in the design of underwater sensor networks. We illustrate some solution strategies that can be implemented using currently available technology, and outline upcoming technological developments that might make underwater wireless sensor networks pervasive in the not-so-distant future. 1.