The literature concerning quartz deformation is reviewed, and the different theories are briefly compared. Two types of experiments have been performed on quartz single crystals in the attempt to produce plastic flow. The highest pressures available in the laboratory (maximum pressure, ca. 138,000 atmospheres) do not produce a measurable degree of plasticity, unless undulatory extinction may be so considered. High-temperature experiments show the tendency of quartz to fracture into needles, governed by the crystallographic properties of the quartz. A very striking correlation is observed between the experimental data and Sander’s observations of major uniformities in his petrofabric studies of S-tectonites. From this is developed a working hypothesis of the mechanism of orientation of quartz in S-tectonites.