The crust and upper mantle encompass a dynamic portion of the Earth system, spanning the transition between the rigid lithosphere and the viscous asthenosphere. Understanding the interactions between these domains and contrasts in properties across them is crucial to assessing the operation of mantle convection, the long-term evolution of tectonic plates and plate boundaries, and the mechanisms underlying natural hazards such as earthquakes and volcanism. A comprehensive view must reconcile theoretical and observational perspectives on the elastic, electromagnetic, gravitational, rheological, thermal and chemical states of the mantle. In this session we seek a range of approaches to investigating the lithosphere and asthenosphere system, including but not limited to geophysical and geological observations, numerical modeling, and laboratory experiments. We especially encourage multidisciplinary contributions that synthesize disparate observations and modeling to make sense of the physical properties and geodynamic behavior of the crust and upper mantle.