This comprehensive treatise reviews, for the first time, all the essential work over the past 160 years on the photoelastic and the closely related linear and quadratic electro-optic effects in isotropic and crystalline mate­ rials. Emphasis is placed on the phenomenal growth of the subject during the past decade and a half with the advent of the laser, with the use of high-frequency acousto-optic and electro-optic techniques, and with the discovery of new piezoelectric materials, all of which have offered a feedback to the wide interest in these two areas of solid-state physics. The first of these subjects, the photoelastic effect, was discovered by Sir David Brewster in 1815. He first found the effect in gels and subsequently found it in glasses and crystals. While the effect remained of academic interest for nearly a hundred years, it became of practical value when Coker and Filon applied it to measuring stresses in machine parts. With one photograph and subsequent analysis, the stress in any planar model can be determined. By taking sections of a three-dimensional model, complete three-dimensional stresses can be found. Hence this effect is widely applied in industry.