During its development years, geophysical survey has served field archaeology by defining possible sites underground, prior to excavation or preservation. Now we can see the art taking off as a research method in its own right. After summarising some recent research applications of magnetic mapping, the author gives us three case studies from USA and Jordan, where ground-penetrating radar (GPR) has produced new interpretations of prehistory and history. Since GPR can map in horizontal slices without damage, it opens up important heritage preservation options. In one case, excavation was discouraged on ethical grounds, in another it was inhibited by the presence of later monuments and in a third, an early agricultural site, the GPR actually saw more than the excavators. This presages a research tool of particular power.