Viscous, gravity-driven ice flow plays a central role in the loss of mass from glaciers and ice sheets, and is underpinned by deformation at the atom to grain scale. Viscous ice flow emerges from the collective motion of crystallographic defects, including vacancies, dislocations, and grain boundaries. The motion of these defects imparts microstructural change—grain size evolution and the development of crystallographic preferred orientation (fabric), for example—that feeds back into the bulk rheological behavior. To improve models of ice mass balance and sea level change, detailed studies of ice rheology and microstructure are thus needed, both of which may evolve through time and space as boundary conditions (e.g., temperature and stress) change. We welcome contributions that examine the rheological and/or microstructural properties of ice across the broad range of temporal and spatial scales, from experimental, modeling, and field-based perspectives. Contributions exploring the rheological properties of extraterrestrial cryospheres are also encouraged.