The previous chapters of this book dealt with phenomena associated with the effects of optical electric fields on the properties of matter. Optical parameters can also be modified by the application of other external stimuli, such as mechanical stress, acoustic waves, low frequency electric fields, and magnetic fields. The study of the effects produced by such stimuli has led to the development of disciplines called acousto-optics, electro-optics, and magnetooptics. Although all of these are of technological significance, the subject of electro-optics has spawned a larger industry and is also more closely related to the subject of nonlinear optics presented in this book. Because electro-optics plays such an important role in the applications of optics today, including many that also employ other aspects of nonlinear optics, it deserves a separate treatment that is given in this chapter.