Magnetization, either spontaneous or field-induced, is always associated with nonreciprocal circular birefringence which breaks the reciprocity principle and qualitatively changes electrodynamics of medium. In magnetic photonic crystals and other periodic structures involving magnetic components, broken reciprocity can result in electromagnetic unidirectionality, when the traveling waves can only propagate in one the two opposite directions. The unidirectional wave propagation can only occur if both time reversal and space inversion symmetries of the periodic structure are broken. During the last decade there have been numerous publications devoted to this kind of phenomenon. Our goal is to present some of those ideas.