The historical record and experimental data on the plasticity of quartz crystals are reviewed, including the discovery of the important phenomenon of hydrolytic weakening. Quartz deforms by slip on numerous planes, generally with Burgers vectors 1/3 ⟨21¿1¿0⟩ and <0001>; ⟨c+a⟩ slip may be activated when resolved shear stresses a and c slip systems are low. Dry natural crystals deformed in an anhydrous environment are very strong, with strengths approaching the intrinsic ''theoretical'' values, at temperatures up to 1000 ¿C. Synthetic crystals with high OH concentrations and dry crystals heat-treated in a hydrogen environment are anomalously weak above a critical temperature (TC) that varies inversely with OH content. There is a transition in the preferred slip systems in crystals equally stressed for a and c slip that coincides with the hydrolytic weakening temperature (Tc). Yielding and flow in all crystals below their respective critical weakening temperatures appear to be controlled by a lattice resistance mechanisms, characterized by a low activation energy and very small activation area. Above the critical temperatures, it appears that crystal flow is facilitated by diffusion of ''water,'' or some related hydroxy defect, to the dislocations, causing hydrolysis of the strong Si-O-Si bonds and aiding bond exchange. This process may assist glide or climb of dislocations-or both-and appears to be rate-limiting. There is evidence that hydrolytic weakening is strongly influenced by confining pressure, possibly through the solubility or diffusivity of H2O or its components in quartz. The change in slip systems at the weakening temperature is attributable to anisotropy of the diffusivity. Similarly, a discontinuity in the creep rates of synthetic crystals, with a concomitant change of activation energy, at the α-β transition is consistent with observed changes in diffusivity at the transition temperature. The experimental data are not yet complete enough to construct an adequate microdynamical model of the flow mechanism.