This chapter leads off with a review of ice crystal structure and the role of dislocations in deformation. Rate-limiting processes are climb at high stresses, grain-boundary slip at intermediate stresses, and diffusion at low stresses. In polycrystalline ice, stress concentrations drive recrystallization by grain growth, polygonization, and nucleation of new grains. The latter and rotation of grains as slip occurs on basal planes leads to preferred orientations of c-axes, and softens the ice. Using a deformation mechanism map in grain size-stress space we show that early experiments spanned the boundary between dislocation creep and creep limited by grain boundary sliding. Much of the deformation in natural ice masses, however, occurs in the latter regime. Next, we introduced Glen’s flow law and related it to these creep mechanisms. Temperature and pressure are incorporated in the flow law by rigorous, physically-based modifications, whereas microfabric and water content must be included empirically. Finally, we introduced linear elastic fracture mechanics and used it to study crevasse depths. Fracturing weakens ice, and this may be included in the flow law with a damage factor.