Abstract This chapter reviews the geophysical data in the U.S. Appalachians–including gravitational and magnetic fields, refraction and reflection seismology, terrestrial heat flow, and electrical properties. An even treatment of the various kinds of geophysical data is neither attempted nor justified. Instead, emphasis and bias are placed primarily on geophysical data that have been useful for the interpretation of the tectonic history of the orogen; of these, seismic reflection data have had the greatest impact on the development and testing of tectonic models in the U.S. Appalachians and elsewhere because of their greater resolution.