Studies of rocks exhumed from paleo-subduction zones (Exhumed Terranes) contribute greatly to our understanding of the materials and process that are hidden beneath the surface in active subduction zones. The goal of this white paper and of ExTerra is to organize research on exhumed terranes in order to accomplish more as a collective than we can as independent researchers. There are three target areas within active subduction zones that are subsequently exposed in exhumed terranes: middle and lower arc crust, the subducted slab, and the mantle wedge. In this document, key scientific questions are identified for each target area along with a summary of relevant recent scientific progress for each question.