Quartz grains occurring in naturally deformed rocks, irrespective of mode of origin, exhibit a wide variety of deformation features, including undulatory extinction, deformation lamellae, fracturing, and marginal granulation. X-ray studies of individual grains show that most of the quartz has deformed plastically by bend gliding. One of the three crystallographic a axes is always the major axis of bending, but no unique glide direction or glide plane has been established. The optical features and the details of the X-ray patterns may be explained and correlated by a theory involving dislocations and polygonizalion, whereby the bent crystal is transformed into a number of elongate, relatively perfect crystallites inclined to one another at small angles and separated by regions of atomic misfit. Undulatory extinction is explained as the optical expression of the results of bend gliding and polygonization. Deformed crystals show a significant decrease in the frequency of Dauphiné twinning; this is also attributed to the bending process.