The interior of the Earth remains our last terra incognita, inaccessible to direct observations. Our understanding of the deformation of the mantle, which shapes our planet through convection and plate tectonics, is based on analysis of: (1) rare mantle rocks carried to the Earth’s surface by volcanic or tectonic processes, (2) the consequences of this deformation on the planet’s surface, and (3) geophysical data. These observables combined with laboratory experiments and numerical modeling imply that olivine deforms via the motion of defects within its crystalline structure and along grain boundaries. Ductile deformation by these crystal-scale processes results in anisotropic propagation of seismic waves, which allows us to probe upper-mantle deformation at scales of tens to hundreds of kilometers.