StrategyPage has a short article about the two latest versions of through-wall radar imagers (namely the Xaver 400 and RadarVision). These devices allow soldiers and police to see objects, including people, on the other side of a wall made of standard building materials, such as concrete, wood, gypsum board (a.k.a. drywall), and the like, but not metal.
These devices work by emitting an RF (radio frequency) radar pulse, listening for the reflection of that pulse off of objects in the wall and on the other side, and using time-delay (i.e. how long it takes for the reflected pulse to come back) and direction (i.e. which direction did the reflection come from) information to construct a picture. This is similar to how bats and dolphins use acoustic echo location, just with an RF pulse instead of sound pulses.
This is a handy capability to have in hostage and other situations where a team has to enter a building where armed attackers or booby traps might be waiting for them. The engineering problems that had to be solved in order to come up with a deployable system were extremely challenging - my had is off to the teams that did this.
PS. In case you are interested, I posted some time ago about a DARPA (Defense Advanced Research Projects Agency, US Department of Defense) effort that developed a through-wall motion detector which was compact and lightweight - so light-weight that you use it with a single hand, like a stud-finder. That post can be found here.