Undulatory extinction of quartz has become important in engineering geology because of its supposed indication of an alkali-silica reactive aggregate. This note reviews the small amount of quantitative data that is available on undulatory extinction of quartz grains in granites and granitic rocks. It is considered that undulatory extinction of quartz occurs as a result of deformation of the rock after its formation and that the greater the cumulative deformation the larger the angle, hence it is a useful indicator of the orogenic history of the rock. Preliminary measurements are reported of the undulatory extinction angle of quartz in some British granites suggesting a field worthy of further study. The note concludes with a discussion of the practical implications of the subject.