An important problem (perhaps the most important problem to the Department of Defense) in modern remote sensing is that of correctly identifying potential targets at great distances and in all kind of weather. Because of their ability to see through clouds and in the absence of ambient radiation, active radar systems are usually required for this task. The practical differences between ground and airborne targets allow the airborne case to focus more on actual imaging (as opposed to clutter rejection) and we will review the current methods within this simpler context. Open problems will be discussed.