This review is concerned with the propagation of electromagnetic waves in magnetically ordered crystals, including the subjects of circular, linear, and elliptic birefringence. New optical phenomena in crystals are analyzed on the basis of magnetic symmetry. Microscopic mechanisms of magneto-optic effects suggested so far are reviewed. Problems in crystal optics of magnetically ordered media are considered. The main results of experimental investigations of magnetic birefringence of light in ferromagnets, ferrimagnets, and antiferromagnets are discussed.