USArray is a dense network of portable and permanent seismic stations that will allow scientists to image the details of Earth structure beneath North America. Over the course of a decade, using a rolling deployment, a transportable array of 400 broadband seismometers will cover the continent with a uniform grid of 2000 sites. As the array moves across the country, 2000 additional sensors will support high-resolution investigations of key geological features. At some sites, special instruments will record electrical and magnetic fields to provide constraints on temperatures and fluid content within the lithosphere. In coordination with the US Geological Survey’s Advanced National Seismic System, observatory-quality permanent stations will be installed to extend the temporal and spatial continuity of earthquake monitoring. By studying the recorded waveforms of hundreds of local, regional and global earthquakes, and large explosions from mines and quarries, scientists will be able to identify and map subtle differences in the velocity and amplitude of seismic energy traveling through Earth. These observations will result in a vastly improved ability to resolve geological structures throughout the entire crust and upper mantle and into Earth’s deepest interior.