Comparison of Moho geometry and reflection character of active collision zones, like the Alps, with those of fossil collision zones, like the Appalachians, lends support for the view that lithospheric delamination is both a common and fundamentally important component of collisional orogeny. In particular, the relative crustal thinning commonly observed beneath the internides of old collisional orogens is an expected consequence of delamination. Neither precollisional crustal thickness variations nor an unrelated postcollision extensional event is required to produce this feature. Acceptance of the delamination hypothesis, in turn, fundamentally alters one's view of the tectonic evolution of old collisional orogens. In the case of the Appalachians, delamination has direct implications for the interpretation of Alleghanian granites, the direction of subduction leading to final closure between Laurentia and Gondwana, and the thermal state of the lithosphere at the onset of Atlantic rifting. A corollary of the delamination hypothesis is that lateral reflectivity variations cannot be used to define paleogeographic terranes in the deep crust of old orogens. Conversely, the observation that delamination leaves a distinctive geophysical imprint on the crust itself provides an important additional clue for deciphering the tectonic evolution of such regions.