The starting point for development of a U.S. program for the Geodynamics Project is twofold. The first is a recognition of the extraordinary advances made during the past five years in our understanding of the origin of earthquakes, volcanism, faulting, and mountain building—the basic processes that shape the surface of our planet. The unifying concept behind these advances has been the group of ideas known as sea-floor spreading, plate tectonics, and global tectonics. It would be difficult to overstate the success of these ideas in bringing together the different disciplines that constitute the earth sciences. The second starting point is the realization that after our initial enthusiasm, it is now time to take a second look at the new ideas. In attemping to fit all of the data of earth science into this new mold, we may fail to recognize equally important new concepts that remain to be discovered.