Acoustic sensors have had a long history of use in military applications. Some of the factors favoring their use are: their ability to exploit loud and distinctive emissions of vehicles and weapons firings, their capability to detect and track targets in non line-of-sight conditions, and the ability to carry out their mission in a totally passive way (no emissions to give out their position). Acoustic-seismic sensors can also be implemented using low-power electronics. Acoustic-seismic sensors are now found in various surveillance sensors, generally known as Unattended Ground Sensors or UGS. The trend towards increasing computational capabilities, lower power consumption and better communications capacity have made these devices more useful and acceptable in a variety of military and peace-keeping operations. The promise of networked sensors has opened the possibility of large-scale sensor networks. However, we must be realistic about what can and cannot be achieved within the current technical horizon. The dream of “sensor dust”: miniature devices, built and deployed at minimum cost, transmitting volumes of data is at present, just that, a dream that has to be tempered with the realities imposed by physics.